test: add Assignment API tests

48 tests covering the XOR validation, the AssetItem.status/active-assignment
preconditions, seat overbooking, seat/status sync across assign+return,
FK-not-found for all three FKs, double-return, 404s, auth-gating, malformed
bodies, and two explicit concurrency tests (Promise.all against a
1-remaining-seat License and against a single in_stock AssetItem) exercising
the transaction race-closing logic in src/lib/assignments.ts.

prisma/migrations/20260803011331_add_assignment/migration.sql

@@ -0,0 +1,22 @@
+-- CreateTable
+CREATE TABLE "assignments" (
+    "id" UUID NOT NULL,
+    "employee_id" UUID NOT NULL,
+    "asset_item_id" UUID,
+    "license_id" UUID,
+    "assigned_at" TIMESTAMPTZ(6) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    "returned_at" TIMESTAMPTZ(6),
+    "signed_at" TIMESTAMPTZ(6),
+    "created_at" TIMESTAMPTZ(6) NOT NULL DEFAULT CURRENT_TIMESTAMP,
+
+    CONSTRAINT "assignments_pkey" PRIMARY KEY ("id")
+);
+
+-- AddForeignKey
+ALTER TABLE "assignments" ADD CONSTRAINT "assignments_employee_id_fkey" FOREIGN KEY ("employee_id") REFERENCES "employees"("id") ON DELETE RESTRICT ON UPDATE CASCADE;
+
+-- AddForeignKey
+ALTER TABLE "assignments" ADD CONSTRAINT "assignments_asset_item_id_fkey" FOREIGN KEY ("asset_item_id") REFERENCES "asset_items"("id") ON DELETE SET NULL ON UPDATE CASCADE;
+
+-- AddForeignKey
+ALTER TABLE "assignments" ADD CONSTRAINT "assignments_license_id_fkey" FOREIGN KEY ("license_id") REFERENCES "licenses"("id") ON DELETE SET NULL ON UPDATE CASCADE;

prisma/schema.prisma

@@ -70,6 +70,10 @@ model Employee {
   // Back-relation for PurchaseOrder.receivedById — an Employee can be the
   // receiver on multiple POs over time.
   purchaseOrdersReceived PurchaseOrder[]
+  // Back-relation for Assignment.employeeId — an Employee can hold multiple
+  // Assignments (assets/licenses) over time, including several active ones
+  // at once (e.g. a laptop + a license seat simultaneously).
+  assignments            Assignment[]
   createdAt              DateTime        @default(now()) @map("created_at") @db.Timestamptz(6)
 
   @@map("employees")
@@ -117,6 +121,10 @@ model AssetItem {
   purchaseOrderId String?         @map("purchase_order_id") @db.Uuid
   purchaseOrder   PurchaseOrder?  @relation(fields: [purchaseOrderId], references: [id])
   createdAt       DateTime        @default(now()) @map("created_at") @db.Timestamptz(6)
+  // Back-relation for Assignment.assetItemId — an AssetItem can have
+  // multiple Assignment rows over its lifetime (historical + at most one
+  // active, see Assignment model's comment), not built until this task.
+  assignments     Assignment[]
 
   @@map("asset_items")
 }
@@ -158,6 +166,10 @@ model License {
   purchaseOrderId String?        @map("purchase_order_id") @db.Uuid
   purchaseOrder   PurchaseOrder? @relation(fields: [purchaseOrderId], references: [id])
   createdAt       DateTime       @default(now()) @map("created_at") @db.Timestamptz(6)
+  // Back-relation for Assignment.licenseId — a License can have multiple
+  // Assignment rows (one per seat handed out, plus historical returns), not
+  // built until this task.
+  assignments     Assignment[]
 
   @@map("licenses")
 }
@@ -247,10 +259,10 @@ enum AssetCategoryValueGroup {
 // (e.g. "Laptop", "Cáp HDMI") freely; only `valueGroup` is a fixed 2-value
 // set.
 model AssetCategory {
-  id            String                       @id @default(uuid()) @db.Uuid
+  id            String                      @id @default(uuid()) @db.Uuid
   name          String
-  valueGroup    AssetCategoryValueGroup      @map("value_group")
+  valueGroup    AssetCategoryValueGroup     @map("value_group")
-  createdAt     DateTime                     @default(now()) @map("created_at") @db.Timestamptz(6)
+  createdAt     DateTime                    @default(now()) @map("created_at") @db.Timestamptz(6)
   attributeDefs AssetCategoryAttributeDef[]
   // Back-relation for AssetModel.categoryId — see "Cập nhật 2026-08-02 (2)"
   // in docs/asset-license-management.md. Only categories with
@@ -344,6 +356,50 @@ model AssetModel {
   @@map("asset_models")
 }
 
+// Assigns an AssetItem (high-value hardware) or a License seat to a specific
+// Employee — see docs/asset-license-management.md's "## Assignment (chốt
+// 2026-08-02)" section and the `Assignment` row in the "Data model (sơ bộ)"
+// table. Exactly one of assetItemId/licenseId must be set (XOR) — enforced
+// at the Zod layer (src/lib/assignments.ts's `.refine()`), NOT a DB
+// constraint, per the same convention as PurchaseOrder's
+// receivedAt/receivedById co-nullability check: Prisma schema can't express
+// an XOR between two nullable FKs natively. Both FKs stay plain nullable
+// columns here for that reason. `assignedAt` defaults to now() but is a
+// real stored column (not derived from createdAt) because a future
+// "transfer" operation (out of scope for this task, see doc) will create a
+// new Assignment row whose assignedAt should reflect the transfer moment,
+// not necessarily row-insert time. `returnedAt` null means still held by
+// the employee — this is also what "active assignment" means throughout
+// the codebase (see the uniqueness rule below). `signedAt` is deliberately
+// just a nullable timestamp marking "handover paperwork signed" — NO file
+// upload, stakeholder explicitly decided against storing a scanned file at
+// this stage (see doc). No `@@unique` on assetItemId here for "only one
+// active assignment at a time" — a plain unique constraint would also
+// forbid a *second, already-returned* historical Assignment for the same
+// AssetItem (e.g. item reassigned after being returned), which is valid and
+// expected. The "at most one ACTIVE (returnedAt IS NULL) assignment per
+// AssetItem" invariant is instead enforced in application code
+// (src/lib/assignments.ts's createAssignment(), inside the same transaction
+// that mutates AssetItem.status/License.seatsUsed) — a partial unique index
+// (`WHERE returned_at IS NULL`) would express this at the DB level too, but
+// that's raw SQL outside a Prisma-expressible `@@unique`, so deliberately
+// not added here; see feature-implementer report for this trade-off.
+model Assignment {
+  id          String     @id @default(uuid()) @db.Uuid
+  employeeId  String     @map("employee_id") @db.Uuid
+  employee    Employee   @relation(fields: [employeeId], references: [id])
+  assetItemId String?    @map("asset_item_id") @db.Uuid
+  assetItem   AssetItem? @relation(fields: [assetItemId], references: [id])
+  licenseId   String?    @map("license_id") @db.Uuid
+  license     License?   @relation(fields: [licenseId], references: [id])
+  assignedAt  DateTime   @default(now()) @map("assigned_at") @db.Timestamptz(6)
+  returnedAt  DateTime?  @map("returned_at") @db.Timestamptz(6)
+  signedAt    DateTime?  @map("signed_at") @db.Timestamptz(6)
+  createdAt   DateTime   @default(now()) @map("created_at") @db.Timestamptz(6)
+
+  @@map("assignments")
+}
+
 // Minimal internal auth accounts — NOT company SSO (deliberately ruled out,
 // see CLAUDE.md / auth decision notes). A handful of trusted IT staff
 // accounts, created by hand via scripts/create-user.ts, no self-registration.

src/app/api/assignments/[id]/route.ts

@@ -0,0 +1,54 @@
+import { NextResponse } from "next/server";
+import { z } from "zod";
+import { auth } from "@/auth";
+import { returnAssignment, validateReturnAssignmentInput } from "@/lib/assignments";
+
+// First dynamic-segment ([id]) route in the codebase — see
+// docs/asset-license-management.md's "Assignment (chốt 2026-08-02)" section,
+// bullet (b): update routes use PATCH /api/<entity>/[id], not an
+// action-based sub-route (e.g. NOT /api/assignments/[id]/return). Next.js
+// 14 App Router route handler params are a plain synchronous object (not a
+// Promise) — that changed starting in Next.js 15, which this project does
+// not use (see package.json / CLAUDE.md).
+export async function PATCH(
+  request: Request,
+  { params }: { params: { id: string } }
+): Promise<NextResponse> {
+  const session = await auth();
+  if (!session) {
+    return NextResponse.json({ error: "Unauthorized" }, { status: 401 });
+  }
+
+  // Validate the [id] path segment's format up front — an invalid UUID
+  // passed straight to Prisma's `findUnique({ where: { id } })` (a `@db.Uuid`
+  // column) throws a raw PrismaClientKnownRequestError that
+  // returnAssignment()'s try/catch doesn't specifically handle, which would
+  // otherwise leak as an uncaught 500 instead of a clean 400.
+  if (!z.string().uuid().safeParse(params.id).success) {
+    return NextResponse.json({ error: "id must be a valid UUID string" }, { status: 400 });
+  }
+
+  let body: unknown;
+  try {
+    body = await request.json();
+  } catch {
+    return NextResponse.json({ error: "Request body must be valid JSON" }, { status: 400 });
+  }
+
+  if (typeof body !== "object" || body === null) {
+    return NextResponse.json({ error: "Request body must be a JSON object" }, { status: 400 });
+  }
+
+  const { returnedAt } = body as Record<string, unknown>;
+  const validation = validateReturnAssignmentInput({ returnedAt });
+
+  if (!validation.ok) {
+    return NextResponse.json({ error: validation.error }, { status: 400 });
+  }
+
+  const updated = await returnAssignment(params.id, validation.value);
+  if (!updated.ok) {
+    return NextResponse.json({ error: updated.error }, { status: updated.status });
+  }
+  return NextResponse.json(updated.value, { status: 200 });
+}

src/app/api/assignments/route.ts

@@ -0,0 +1,51 @@
+import { NextResponse } from "next/server";
+import { auth } from "@/auth";
+import {
+  createAssignment,
+  listAssignments,
+  validateCreateAssignmentInput,
+} from "@/lib/assignments";
+
+// Mirrors src/app/api/licenses/route.ts's pattern — middleware.ts excludes
+// /api from route protection, so this checks the session itself. Flat
+// permission model (CLAUDE.md): only "logged in or not", no role check.
+export async function POST(request: Request): Promise<NextResponse> {
+  const session = await auth();
+  if (!session) {
+    return NextResponse.json({ error: "Unauthorized" }, { status: 401 });
+  }
+
+  let body: unknown;
+  try {
+    body = await request.json();
+  } catch {
+    return NextResponse.json({ error: "Request body must be valid JSON" }, { status: 400 });
+  }
+
+  if (typeof body !== "object" || body === null) {
+    return NextResponse.json({ error: "Request body must be a JSON object" }, { status: 400 });
+  }
+
+  const { employeeId, assetItemId, licenseId } = body as Record<string, unknown>;
+  const validation = validateCreateAssignmentInput({ employeeId, assetItemId, licenseId });
+
+  if (!validation.ok) {
+    return NextResponse.json({ error: validation.error }, { status: 400 });
+  }
+
+  const created = await createAssignment(validation.value);
+  if (!created.ok) {
+    return NextResponse.json({ error: created.error }, { status: 400 });
+  }
+  return NextResponse.json(created.value, { status: 201 });
+}
+
+export async function GET(): Promise<NextResponse> {
+  const session = await auth();
+  if (!session) {
+    return NextResponse.json({ error: "Unauthorized" }, { status: 401 });
+  }
+
+  const assignments = await listAssignments();
+  return NextResponse.json(assignments, { status: 200 });
+}

src/lib/assignments.ts

@@ -0,0 +1,409 @@
+import { z } from "zod";
+import { Prisma } from "@prisma/client";
+import { prisma } from "./prisma";
+
+export interface AssignmentRecord {
+  id: string;
+  employee_id: string;
+  asset_item_id: string | null;
+  license_id: string | null;
+  assigned_at: string;
+  returned_at: string | null;
+  signed_at: string | null;
+  created_at: string;
+}
+
+// assignedAt/returnedAt/signedAt are all deliberately NOT accepted from the
+// client at creation time — same precedent as AssetItem.status always
+// starting "in_stock" and License.seatsUsed always starting 0:
+// assignedAt is a system-set "moment of assignment" timestamp
+// (Prisma's @default(now()) on Assignment.assignedAt), returnedAt must be
+// null on creation (an Assignment cannot be created already-returned — that
+// would make no sense given the seat/status side-effects below), and
+// signedAt (marking handover paperwork as signed) is not wired to any
+// endpoint in this task at all — see feature-implementer report's "Out of
+// scope" section. It stays null until a future "mark as signed" operation
+// is built.
+export interface CreateAssignmentInput {
+  employeeId: string;
+  assetItemId?: string | null;
+  licenseId?: string | null;
+}
+
+export type ValidationResult =
+  | { ok: true; value: CreateAssignmentInput }
+  | { ok: false; error: string };
+
+// Mirrors src/lib/purchase-orders.ts's dateString helper — accepts any
+// string Date.parse() can handle and transforms it into a Date for Prisma.
+function dateString(message: string) {
+  return z
+    .string({ required_error: message, invalid_type_error: message })
+    .refine((value) => !Number.isNaN(Date.parse(value)), { message })
+    .transform((value) => new Date(value));
+}
+
+const createAssignmentSchema = z
+  .object({
+    // FK to Employee — only a UUID *format* check here, whether it
+    // references a real employee is left to Prisma's own FK constraint at
+    // insert time (see createAssignment()'s P2003 handling below). Mirrors
+    // src/lib/requests.ts's employee_id handling.
+    employeeId: z
+      .string({ required_error: "employeeId is required and must be a valid UUID string" })
+      .uuid("employeeId is required and must be a valid UUID string"),
+    // Exactly one of assetItemId/licenseId must be set — see the .refine()
+    // below for the XOR check. Unlike other nullable FKs in this codebase
+    // (e.g. AssetItem.purchaseOrderId), whether these reference a real row
+    // is NOT deferred entirely to Prisma's insert-time FK constraint —
+    // createAssignment() needs to read the referenced row anyway (to check
+    // AssetItem.status / License.seatsUsed business rules), so existence is
+    // checked explicitly inside the transaction instead. Only format
+    // validation happens here at the Zod layer.
+    assetItemId: z
+      .string({ invalid_type_error: "assetItemId must be a valid UUID string" })
+      .uuid("assetItemId must be a valid UUID string")
+      .optional()
+      .nullable(),
+    licenseId: z
+      .string({ invalid_type_error: "licenseId must be a valid UUID string" })
+      .uuid("licenseId must be a valid UUID string")
+      .optional()
+      .nullable(),
+  })
+  .refine(
+    (value) => {
+      const hasAssetItem = value.assetItemId !== undefined && value.assetItemId !== null;
+      const hasLicense = value.licenseId !== undefined && value.licenseId !== null;
+      // XOR: exactly one of the two must be set.
+      return hasAssetItem !== hasLicense;
+    },
+    {
+      message: "Exactly one of assetItemId or licenseId must be set (not both, not neither)",
+      // Attach the error to both fields so it surfaces regardless of which
+      // one the caller looks at first — mirrors
+      // src/lib/purchase-orders.ts's receivedAt/receivedById co-nullability
+      // refine, which attaches to a single path for the same reason.
+      path: ["assetItemId"],
+    }
+  );
+
+/**
+ * Validates raw (untyped) assignment-creation input. employeeId must be a
+ * valid UUID string. Exactly one of assetItemId/licenseId must be a valid
+ * UUID string (XOR — both set or both unset is rejected). assignedAt,
+ * returnedAt, and signedAt are not accepted here — see CreateAssignmentInput's
+ * doc comment for why.
+ */
+export function validateCreateAssignmentInput(input: {
+  employeeId?: unknown;
+  assetItemId?: unknown;
+  licenseId?: unknown;
+}): ValidationResult {
+  const result = createAssignmentSchema.safeParse(input);
+  if (!result.success) {
+    const message = result.error.issues[0]?.message ?? "Invalid input";
+    return { ok: false, error: message };
+  }
+  return { ok: true, value: result.data };
+}
+
+const returnAssignmentSchema = z.object({
+  // Optional — defaults to "now" at the time returnAssignment() runs if not
+  // provided (mirrors Assignment.assignedAt's own default-now behavior at
+  // creation). Accepting an explicit value lets a caller backdate a return
+  // (e.g. recording a return that was physically handled yesterday but only
+  // entered into the system today) — same spirit as PurchaseOrder accepting
+  // an explicit receivedAt rather than always stamping "now".
+  returnedAt: dateString("returnedAt must be a valid date string").optional(),
+});
+
+export type ReturnValidationResult =
+  | { ok: true; value: { returnedAt?: Date } }
+  | { ok: false; error: string };
+
+/**
+ * Validates raw (untyped) assignment-return input. returnedAt is optional
+ * but must be a valid date string if present.
+ */
+export function validateReturnAssignmentInput(input: {
+  returnedAt?: unknown;
+}): ReturnValidationResult {
+  const result = returnAssignmentSchema.safeParse(input);
+  if (!result.success) {
+    const message = result.error.issues[0]?.message ?? "Invalid input";
+    return { ok: false, error: message };
+  }
+  return { ok: true, value: result.data };
+}
+
+function toRecord(row: {
+  id: string;
+  employeeId: string;
+  assetItemId: string | null;
+  licenseId: string | null;
+  assignedAt: Date;
+  returnedAt: Date | null;
+  signedAt: Date | null;
+  createdAt: Date;
+}): AssignmentRecord {
+  return {
+    id: row.id,
+    employee_id: row.employeeId,
+    asset_item_id: row.assetItemId,
+    license_id: row.licenseId,
+    assigned_at: row.assignedAt.toISOString(),
+    returned_at: row.returnedAt ? row.returnedAt.toISOString() : null,
+    signed_at: row.signedAt ? row.signedAt.toISOString() : null,
+    created_at: row.createdAt.toISOString(),
+  };
+}
+
+// Internal error type used to carry a clean, already-user-facing message out
+// of the `prisma.$transaction()` callback below, distinguishing
+// application-level business-rule rejections (400) from unexpected/infra
+// errors (rethrown as-is). Not exported — callers only ever see the
+// {ok,error} result shape.
+class AssignmentBusinessError extends Error {
+  constructor(message: string) {
+    super(message);
+    this.name = "AssignmentBusinessError";
+  }
+}
+
+class AssignmentNotFoundError extends Error {
+  constructor() {
+    super("Assignment not found");
+    this.name = "AssignmentNotFoundError";
+  }
+}
+
+export type CreateAssignmentResult =
+  | { ok: true; value: AssignmentRecord }
+  | { ok: false; error: string };
+
+/**
+ * Creates an Assignment (assigns an AssetItem or a License seat to an
+ * Employee), enforcing every business rule from docs/
+ * asset-license-management.md's "## Assignment (chốt 2026-08-02)" section
+ * inside a single `prisma.$transaction()`:
+ *
+ * - AssetItem branch: rejects if the AssetItem doesn't exist, rejects if its
+ *   current status isn't "in_stock" (stakeholder-confirmed precondition —
+ *   see the doc's "chốt 2026-08-02 (2)" note), rejects if there's already an
+ *   ACTIVE Assignment (returnedAt IS NULL) for it (explicit
+ *   belt-and-suspenders invariant per the doc, on top of the status check),
+ *   then atomically flips AssetItem.status to "assigned".
+ * - License branch: rejects if the License doesn't exist, rejects if
+ *   seatsUsed >= seatsTotal (no overbooking), then atomically increments
+ *   seatsUsed by 1.
+ *
+ * Race-condition note (read this before touching the transaction body):
+ * both branches use a two-step "read for validation, then conditional
+ * `updateMany` with a WHERE guard" pattern instead of a plain read-then-write
+ * (`findUnique` then `update`). The WHERE-guarded `updateMany` — e.g.
+ * `updateMany({ where: { id, status: "in_stock" }, data: { status:
+ * "assigned" } })` — is the actual race-closing mechanism: Postgres
+ * evaluates and applies that as a single atomic UPDATE statement, taking a
+ * row lock for its duration. Two concurrent transactions racing on the same
+ * AssetItem/License row serialize on that lock — the first to commit wins,
+ * and the second's WHERE clause is re-evaluated against the now-committed
+ * row and no longer matches, so its `updateMany` returns `count: 0`, which
+ * this function treats as a "lost the race" rejection. This is why the
+ * `findUnique` calls right before each `updateMany` are NOT the
+ * race-closing check by themselves (there's a real TOCTOU gap between a
+ * `findUnique` and a later `update` in isolation) — they only exist to
+ * produce a specific, friendly error message (not-found vs. wrong-status vs.
+ * no-seats) for the *common* (non-racing) case; the `updateMany`'s `count`
+ * is always re-checked afterwards and is what actually gets trusted.
+ * seatsTotal itself is read once via that same `findUnique` and reused in
+ * the `updateMany`'s WHERE clause (`seatsUsed: { lt: seatsTotal }`) — this
+ * is safe because nothing in this codebase ever mutates seatsTotal after
+ * creation (no update endpoint exists for it), so it can't go stale between
+ * the read and the write within the same transaction.
+ *
+ * The explicit "no existing active Assignment for this AssetItem" check
+ * (`tx.assignment.findFirst({ where: { assetItemId, returnedAt: null } })`)
+ * is intentionally layered BEFORE the atomic status update, as defense in
+ * depth against AssetItem.status/Assignment-row drift (e.g. manual DB
+ * intervention) — under normal operation this codebase always keeps
+ * `status === "assigned"` in exact sync with "has an active Assignment", so
+ * the atomic status-guarded `updateMany` above is what actually closes the
+ * concurrency race for the AssetItem branch; this check does not need its
+ * own atomic guard on top because the transaction's row lock (from the
+ * subsequent `updateMany`) is what makes the overall operation safe, not
+ * this read on its own.
+ */
+export async function createAssignment(
+  input: CreateAssignmentInput
+): Promise<CreateAssignmentResult> {
+  try {
+    const created = await prisma.$transaction(async (tx) => {
+      if (input.assetItemId) {
+        const assetItem = await tx.assetItem.findUnique({ where: { id: input.assetItemId } });
+        if (!assetItem) {
+          throw new AssignmentBusinessError(
+            "assetItemId does not reference an existing asset item"
+          );
+        }
+        if (assetItem.status !== "in_stock") {
+          throw new AssignmentBusinessError(
+            `assetItemId refers to an AssetItem that is not in_stock (current status: ${assetItem.status})`
+          );
+        }
+        const existingActive = await tx.assignment.findFirst({
+          where: { assetItemId: input.assetItemId, returnedAt: null },
+        });
+        if (existingActive) {
+          throw new AssignmentBusinessError(
+            "assetItemId already has an active assignment (returnedAt IS NULL)"
+          );
+        }
+
+        // Atomic race-closing gate — see this function's doc comment.
+        const statusUpdate = await tx.assetItem.updateMany({
+          where: { id: input.assetItemId, status: "in_stock" },
+          data: { status: "assigned" },
+        });
+        if (statusUpdate.count === 0) {
+          throw new AssignmentBusinessError(
+            "assetItemId refers to an AssetItem that is not in_stock (lost a race to another concurrent assignment)"
+          );
+        }
+      }
+
+      if (input.licenseId) {
+        const license = await tx.license.findUnique({ where: { id: input.licenseId } });
+        if (!license) {
+          throw new AssignmentBusinessError("licenseId does not reference an existing license");
+        }
+        if (license.seatsUsed >= license.seatsTotal) {
+          throw new AssignmentBusinessError(
+            "licenseId has no seats remaining (seatsUsed >= seatsTotal)"
+          );
+        }
+
+        // Atomic race-closing gate — see this function's doc comment.
+        const seatUpdate = await tx.license.updateMany({
+          where: { id: input.licenseId, seatsUsed: { lt: license.seatsTotal } },
+          data: { seatsUsed: { increment: 1 } },
+        });
+        if (seatUpdate.count === 0) {
+          throw new AssignmentBusinessError(
+            "licenseId has no seats remaining (lost a race to another concurrent assignment)"
+          );
+        }
+      }
+
+      return tx.assignment.create({
+        data: {
+          employeeId: input.employeeId,
+          assetItemId: input.assetItemId ?? null,
+          licenseId: input.licenseId ?? null,
+        },
+      });
+    });
+
+    return { ok: true, value: toRecord(created) };
+  } catch (error) {
+    if (error instanceof AssignmentBusinessError) {
+      return { ok: false, error: error.message };
+    }
+    // Only employeeId can still fail with a raw FK violation here —
+    // assetItemId/licenseId existence is already fully resolved above
+    // (found-or-rejected before the transaction ever reaches
+    // tx.assignment.create()).
+    if (error instanceof Prisma.PrismaClientKnownRequestError && error.code === "P2003") {
+      return { ok: false, error: "employeeId does not reference an existing employee" };
+    }
+    throw error;
+  }
+}
+
+export async function listAssignments(): Promise<AssignmentRecord[]> {
+  const rows = await prisma.assignment.findMany({
+    orderBy: [{ createdAt: "desc" }, { id: "desc" }],
+  });
+  return rows.map(toRecord);
+}
+
+export type ReturnAssignmentResult =
+  | { ok: true; value: AssignmentRecord }
+  | { ok: false; status: 404 | 400; error: string };
+
+/**
+ * Marks an Assignment as returned (sets returnedAt), and reverses its
+ * side-effects from createAssignment() inside the same transaction:
+ * - If assetItemId is set: flips AssetItem.status back to "in_stock" —
+ *   always, regardless of any RMA/repair path (there isn't one built yet,
+ *   per the task's instructions; see feature-implementer report).
+ * - If licenseId is set: atomically decrements License.seatsUsed by 1.
+ *
+ * Race-condition note: double-return is closed the same way as
+ * createAssignment()'s races — an atomic, WHERE-guarded `updateMany`
+ * (`where: { id, returnedAt: null }`) is the actual gate. Two concurrent
+ * PATCH-return calls against the same Assignment race on that row's UPDATE
+ * lock; only the first to commit gets `count: 1`, the second gets `count: 0`
+ * and is rejected as "already returned". The initial `findUnique` (used to
+ * read assetItemId/licenseId for the AssetItem/License side-effects, and to
+ * distinguish "not found" from "already returned" in the common case) is
+ * NOT itself the race-closing check, same caveat as createAssignment()'s
+ * doc comment. Decrementing seatsUsed does not need its own conditional
+ * `updateMany` guard the way incrementing did on the assign side — a bare
+ * `{ decrement: 1 }` is a single atomic UPDATE with no upper bound to
+ * enforce (unlike assign's seatsUsed < seatsTotal check), so a plain
+ * `update()` is race-safe on its own here.
+ */
+export async function returnAssignment(
+  id: string,
+  input: { returnedAt?: Date }
+): Promise<ReturnAssignmentResult> {
+  try {
+    const updated = await prisma.$transaction(async (tx) => {
+      const assignment = await tx.assignment.findUnique({ where: { id } });
+      if (!assignment) {
+        throw new AssignmentNotFoundError();
+      }
+
+      const returnedAt = input.returnedAt ?? new Date();
+
+      // Atomic race-closing gate — see this function's doc comment. Also
+      // doubles as the "already returned" rejection in the non-racing case,
+      // since `assignment.returnedAt !== null` implies this WHERE clause
+      // won't match either.
+      const claim = await tx.assignment.updateMany({
+        where: { id, returnedAt: null },
+        data: { returnedAt },
+      });
+      if (claim.count === 0) {
+        throw new AssignmentBusinessError("Assignment has already been returned");
+      }
+
+      if (assignment.assetItemId) {
+        await tx.assetItem.update({
+          where: { id: assignment.assetItemId },
+          data: { status: "in_stock" },
+        });
+      }
+
+      if (assignment.licenseId) {
+        await tx.license.update({
+          where: { id: assignment.licenseId },
+          data: { seatsUsed: { decrement: 1 } },
+        });
+      }
+
+      return tx.assignment.findUniqueOrThrow({ where: { id } });
+    });
+
+    return { ok: true, value: toRecord(updated) };
+  } catch (error) {
+    if (error instanceof AssignmentNotFoundError) {
+      return { ok: false, status: 404, error: "Assignment not found" };
+    }
+    if (error instanceof AssignmentBusinessError) {
+      return { ok: false, status: 400, error: error.message };
+    }
+    throw error;
+  }
+}

tests/assignments.test.ts

@@ -0,0 +1,764 @@
+import { beforeEach, describe, expect, it, vi } from "vitest";
+import type { Mock } from "vitest";
+import type { Session } from "next-auth";
+import {
+  createAssignment,
+  listAssignments,
+  returnAssignment,
+  validateCreateAssignmentInput,
+  validateReturnAssignmentInput,
+} from "@/lib/assignments";
+import { prisma } from "@/lib/prisma";
+
+// This file has two layers of coverage, mirroring tests/licenses.test.ts:
+// - lib-level tests exercise src/lib/assignments.ts directly (validation,
+//   transactional business rules, race conditions).
+// - route-level tests exercise src/app/api/assignments/route.ts and
+//   src/app/api/assignments/[id]/route.ts over HTTP, including the auth()
+//   gate — mirrors tests/licenses.test.ts's @/auth mocking pattern.
+vi.mock("@/auth", () => ({ auth: vi.fn() }));
+
+import { GET, POST } from "@/app/api/assignments/route";
+import { PATCH } from "@/app/api/assignments/[id]/route";
+import { auth } from "@/auth";
+
+const mockedAuth = auth as unknown as Mock<() => Promise<Session | null>>;
+
+const testSession: Session = {
+  user: { id: "test-user-id" },
+  expires: new Date(Date.now() + 60 * 60 * 1000).toISOString(),
+};
+
+function postRequest(body: unknown): Request {
+  return new Request("http://localhost/api/assignments", {
+    method: "POST",
+    headers: { "content-type": "application/json" },
+    body: JSON.stringify(body),
+  });
+}
+
+function patchRequest(body: unknown): Request {
+  return new Request("http://localhost/api/assignments/x", {
+    method: "PATCH",
+    headers: { "content-type": "application/json" },
+    body: JSON.stringify(body),
+  });
+}
+
+// Assignment.employeeId is a real FK to Employee — mirrors
+// tests/requests.test.ts's createTestEmployee() helper.
+function createTestEmployee(overrides: { name?: string } = {}) {
+  return prisma.employee.create({
+    data: {
+      name: overrides.name ?? "Test Employee",
+      department: "IT",
+    },
+  });
+}
+
+// Assignment.assetItemId is a real FK to AssetItem, which itself requires a
+// full AssetModel -> AssetCategory chain. Mirrors tests/assets.test.ts's
+// createTestModelId() helper.
+async function createTestModelId(): Promise<string> {
+  const category = await prisma.assetCategory.create({
+    data: { name: "Laptop", valueGroup: "high_value" },
+  });
+  const model = await prisma.assetModel.create({
+    data: { categoryId: category.id, manufacturer: "Dell", name: "Latitude 5440" },
+  });
+  return model.id;
+}
+
+let serialCounter = 0;
+function createTestAssetItem(overrides: { modelId: string; status?: "in_stock" | "assigned" | "repair" | "retired" }) {
+  serialCounter += 1;
+  return prisma.assetItem.create({
+    data: {
+      serialNumber: `SN-ASSIGN-${serialCounter}`,
+      modelId: overrides.modelId,
+      purchaseDate: new Date("2026-01-15"),
+      status: overrides.status ?? "in_stock",
+    },
+  });
+}
+
+// Assignment.licenseId is a real FK to License, which itself requires a
+// Vendor. Mirrors tests/licenses.test.ts's createTestVendor() helper.
+function createTestVendor(overrides: { name?: string } = {}) {
+  return prisma.vendor.create({
+    data: {
+      name: overrides.name ?? "Test Vendor",
+      contactEmail: "[email protected]",
+      contactPhone: "0281234567",
+      address: "123 Test Street",
+    },
+  });
+}
+
+function createTestLicense(overrides: {
+  vendorId: string;
+  seatsTotal?: number;
+  seatsUsed?: number;
+}) {
+  return prisma.license.create({
+    data: {
+      name: "Adobe Creative Cloud",
+      vendorId: overrides.vendorId,
+      seatsTotal: overrides.seatsTotal ?? 5,
+      seatsUsed: overrides.seatsUsed ?? 0,
+      licenseType: "subscription",
+    },
+  });
+}
+
+async function createTestLicenseId(overrides: { seatsTotal?: number; seatsUsed?: number } = {}): Promise<string> {
+  const vendor = await createTestVendor();
+  const license = await createTestLicense({ vendorId: vendor.id, ...overrides });
+  return license.id;
+}
+
+beforeEach(async () => {
+  // Clear in FK-safe order: assignments first (references everything else),
+  // then asset_items/licenses (reference purchase_orders/vendors/models),
+  // then the rest.
+  await prisma.assignment.deleteMany();
+  await prisma.assetItem.deleteMany();
+  await prisma.license.deleteMany();
+  await prisma.assetModel.deleteMany();
+  await prisma.assetCategory.deleteMany();
+  await prisma.purchaseOrder.deleteMany();
+  await prisma.vendor.deleteMany();
+  await prisma.employee.deleteMany();
+  serialCounter = 0;
+  // Default to a logged-in session — the lib-level tests don't go through
+  // the route at all (no-op for them), and the route-level tests exercise
+  // business logic, not auth gating (covered by the "auth gating" block).
+  mockedAuth.mockResolvedValue(testSession);
+});
+
+describe("validateCreateAssignmentInput", () => {
+  it("accepts input with only assetItemId set", () => {
+    const result = validateCreateAssignmentInput({
+      employeeId: "11111111-1111-1111-1111-111111111111",
+      assetItemId: "22222222-2222-2222-2222-222222222222",
+    });
+    expect(result.ok).toBe(true);
+  });
+
+  it("accepts input with only licenseId set", () => {
+    const result = validateCreateAssignmentInput({
+      employeeId: "11111111-1111-1111-1111-111111111111",
+      licenseId: "33333333-3333-3333-3333-333333333333",
+    });
+    expect(result.ok).toBe(true);
+  });
+
+  it("rejects input with neither assetItemId nor licenseId set", () => {
+    const result = validateCreateAssignmentInput({
+      employeeId: "11111111-1111-1111-1111-111111111111",
+    });
+    expect(result.ok).toBe(false);
+    if (!result.ok) {
+      expect(result.error).toMatch(/exactly one/i);
+    }
+  });
+
+  it("rejects input with both assetItemId and licenseId set", () => {
+    const result = validateCreateAssignmentInput({
+      employeeId: "11111111-1111-1111-1111-111111111111",
+      assetItemId: "22222222-2222-2222-2222-222222222222",
+      licenseId: "33333333-3333-3333-3333-333333333333",
+    });
+    expect(result.ok).toBe(false);
+    if (!result.ok) {
+      expect(result.error).toMatch(/exactly one/i);
+    }
+  });
+
+  it("rejects a missing employeeId", () => {
+    const result = validateCreateAssignmentInput({
+      assetItemId: "22222222-2222-2222-2222-222222222222",
+    });
+    expect(result.ok).toBe(false);
+    if (!result.ok) {
+      expect(result.error).toMatch(/employeeId/i);
+    }
+  });
+
+  it("rejects a non-UUID assetItemId", () => {
+    const result = validateCreateAssignmentInput({
+      employeeId: "11111111-1111-1111-1111-111111111111",
+      assetItemId: "not-a-uuid",
+    });
+    expect(result.ok).toBe(false);
+    if (!result.ok) {
+      expect(result.error).toMatch(/assetItemId/i);
+    }
+  });
+});
+
+describe("validateReturnAssignmentInput", () => {
+  it("accepts an empty body (returnedAt defaults to now later)", () => {
+    const result = validateReturnAssignmentInput({});
+    expect(result.ok).toBe(true);
+    if (result.ok) {
+      expect(result.value.returnedAt).toBeUndefined();
+    }
+  });
+
+  it("accepts an explicit returnedAt", () => {
+    const result = validateReturnAssignmentInput({ returnedAt: "2026-08-05" });
+    expect(result.ok).toBe(true);
+    if (result.ok) {
+      expect(result.value.returnedAt).toBeInstanceOf(Date);
+    }
+  });
+
+  it("rejects an invalid returnedAt", () => {
+    const result = validateReturnAssignmentInput({ returnedAt: "not-a-date" });
+    expect(result.ok).toBe(false);
+  });
+});
+
+describe("createAssignment", () => {
+  it("assigns an in_stock AssetItem, flips its status to assigned", async () => {
+    const employee = await createTestEmployee();
+    const modelId = await createTestModelId();
+    const assetItem = await createTestAssetItem({ modelId });
+
+    const created = await createAssignment({ employeeId: employee.id, assetItemId: assetItem.id });
+    expect(created.ok).toBe(true);
+    if (!created.ok) return;
+    expect(created.value.asset_item_id).toBe(assetItem.id);
+    expect(created.value.license_id).toBeNull();
+    expect(created.value.returned_at).toBeNull();
+
+    const updatedItem = await prisma.assetItem.findUniqueOrThrow({ where: { id: assetItem.id } });
+    expect(updatedItem.status).toBe("assigned");
+  });
+
+  it.each(["repair", "retired", "assigned"] as const)(
+    "rejects assigning an AssetItem whose status is %s",
+    async (status) => {
+      const employee = await createTestEmployee();
+      const modelId = await createTestModelId();
+      const assetItem = await createTestAssetItem({ modelId, status });
+
+      const created = await createAssignment({ employeeId: employee.id, assetItemId: assetItem.id });
+      expect(created.ok).toBe(false);
+      if (created.ok) return;
+      expect(created.error).toMatch(/not in_stock/i);
+
+      const remaining = await prisma.assignment.findMany();
+      expect(remaining).toHaveLength(0);
+    }
+  );
+
+  it("rejects assigning an AssetItem that already has an active assignment (belt-and-suspenders check)", async () => {
+    const employee = await createTestEmployee();
+    const modelId = await createTestModelId();
+    const assetItem = await createTestAssetItem({ modelId });
+
+    const first = await createAssignment({ employeeId: employee.id, assetItemId: assetItem.id });
+    expect(first.ok).toBe(true);
+
+    // Manually force status back to in_stock without closing the active
+    // Assignment row, to exercise the explicit "active assignment already
+    // exists" invariant independently of the status check (data-drift
+    // scenario per src/lib/assignments.ts's doc comment).
+    await prisma.assetItem.update({ where: { id: assetItem.id }, data: { status: "in_stock" } });
+
+    const second = await createAssignment({ employeeId: employee.id, assetItemId: assetItem.id });
+    expect(second.ok).toBe(false);
+    if (second.ok) return;
+    expect(second.error).toMatch(/already has an active assignment/i);
+  });
+
+  it("rejects assigning an AssetItem for a second employee while the first assignment is still active", async () => {
+    const employee1 = await createTestEmployee({ name: "Employee 1" });
+    const employee2 = await createTestEmployee({ name: "Employee 2" });
+    const modelId = await createTestModelId();
+    const assetItem = await createTestAssetItem({ modelId });
+
+    const first = await createAssignment({ employeeId: employee1.id, assetItemId: assetItem.id });
+    expect(first.ok).toBe(true);
+
+    const second = await createAssignment({ employeeId: employee2.id, assetItemId: assetItem.id });
+    expect(second.ok).toBe(false);
+    if (second.ok) return;
+    expect(second.error).toMatch(/not in_stock/i);
+
+    const activeAssignments = await prisma.assignment.findMany({ where: { assetItemId: assetItem.id, returnedAt: null } });
+    expect(activeAssignments).toHaveLength(1);
+  });
+
+  it("allows re-assigning an AssetItem to a new employee after it has been returned", async () => {
+    const employee1 = await createTestEmployee({ name: "Employee 1" });
+    const employee2 = await createTestEmployee({ name: "Employee 2" });
+    const modelId = await createTestModelId();
+    const assetItem = await createTestAssetItem({ modelId });
+
+    const first = await createAssignment({ employeeId: employee1.id, assetItemId: assetItem.id });
+    expect(first.ok).toBe(true);
+    if (!first.ok) return;
+    const returned = await returnAssignment(first.value.id, {});
+    expect(returned.ok).toBe(true);
+
+    const second = await createAssignment({ employeeId: employee2.id, assetItemId: assetItem.id });
+    expect(second.ok).toBe(true);
+    if (!second.ok) return;
+    expect(second.value.employee_id).toBe(employee2.id);
+
+    const updatedItem = await prisma.assetItem.findUniqueOrThrow({ where: { id: assetItem.id } });
+    expect(updatedItem.status).toBe("assigned");
+  });
+
+  it("returns a clean error when assetItemId doesn't reference an existing asset item", async () => {
+    const employee = await createTestEmployee();
+    const created = await createAssignment({
+      employeeId: employee.id,
+      assetItemId: "00000000-0000-0000-0000-000000000000",
+    });
+    expect(created.ok).toBe(false);
+    if (created.ok) return;
+    expect(created.error).toMatch(/assetItemId/i);
+    expect(created.error).toMatch(/does not reference an existing asset item/i);
+  });
+
+  it("returns a clean error when employeeId doesn't reference an existing employee", async () => {
+    const modelId = await createTestModelId();
+    const assetItem = await createTestAssetItem({ modelId });
+
+    const created = await createAssignment({
+      employeeId: "00000000-0000-0000-0000-000000000000",
+      assetItemId: assetItem.id,
+    });
+    expect(created.ok).toBe(false);
+    if (created.ok) return;
+    expect(created.error).toMatch(/employeeId/i);
+    expect(created.error).toMatch(/does not reference an existing employee/i);
+
+    // And the AssetItem's status must NOT have been mutated — the
+    // transaction must have rolled back entirely.
+    const item = await prisma.assetItem.findUniqueOrThrow({ where: { id: assetItem.id } });
+    expect(item.status).toBe("in_stock");
+  });
+
+  it("assigns a license seat, increments seatsUsed", async () => {
+    const employee = await createTestEmployee();
+    const licenseId = await createTestLicenseId({ seatsTotal: 5, seatsUsed: 2 });
+
+    const created = await createAssignment({ employeeId: employee.id, licenseId });
+    expect(created.ok).toBe(true);
+    if (!created.ok) return;
+    expect(created.value.license_id).toBe(licenseId);
+
+    const license = await prisma.license.findUniqueOrThrow({ where: { id: licenseId } });
+    expect(license.seatsUsed).toBe(3);
+  });
+
+  it("rejects assigning a license with no seats remaining (seatsUsed >= seatsTotal)", async () => {
+    const employee = await createTestEmployee();
+    const licenseId = await createTestLicenseId({ seatsTotal: 3, seatsUsed: 3 });
+
+    const created = await createAssignment({ employeeId: employee.id, licenseId });
+    expect(created.ok).toBe(false);
+    if (created.ok) return;
+    expect(created.error).toMatch(/no seats remaining/i);
+
+    const license = await prisma.license.findUniqueOrThrow({ where: { id: licenseId } });
+    expect(license.seatsUsed).toBe(3);
+    const remaining = await prisma.assignment.findMany();
+    expect(remaining).toHaveLength(0);
+  });
+
+  it("returns a clean error when licenseId doesn't reference an existing license", async () => {
+    const employee = await createTestEmployee();
+    const created = await createAssignment({
+      employeeId: employee.id,
+      licenseId: "00000000-0000-0000-0000-000000000000",
+    });
+    expect(created.ok).toBe(false);
+    if (created.ok) return;
+    expect(created.error).toMatch(/licenseId/i);
+    expect(created.error).toMatch(/does not reference an existing license/i);
+  });
+
+  it("closes the race: two concurrent createAssignment calls against a license with exactly 1 remaining seat — at least one must fail, seatsUsed never exceeds seatsTotal", async () => {
+    const employee1 = await createTestEmployee({ name: "Racer 1" });
+    const employee2 = await createTestEmployee({ name: "Racer 2" });
+    // seatsTotal=3, seatsUsed=2 -> exactly 1 seat remaining.
+    const licenseId = await createTestLicenseId({ seatsTotal: 3, seatsUsed: 2 });
+
+    const [resultA, resultB] = await Promise.all([
+      createAssignment({ employeeId: employee1.id, licenseId }),
+      createAssignment({ employeeId: employee2.id, licenseId }),
+    ]);
+
+    const outcomes = [resultA, resultB];
+    const succeeded = outcomes.filter((r) => r.ok);
+    const failed = outcomes.filter((r) => !r.ok);
+
+    expect(succeeded).toHaveLength(1);
+    expect(failed).toHaveLength(1);
+    if (!failed[0].ok) {
+      expect(failed[0].error).toMatch(/no seats remaining/i);
+    }
+
+    const license = await prisma.license.findUniqueOrThrow({ where: { id: licenseId } });
+    expect(license.seatsUsed).toBe(3);
+    expect(license.seatsUsed).toBeLessThanOrEqual(license.seatsTotal);
+
+    const assignments = await prisma.assignment.findMany({ where: { licenseId } });
+    expect(assignments).toHaveLength(1);
+  });
+
+  it("closes the race: two concurrent createAssignment calls against the same in_stock AssetItem — at least one must fail", async () => {
+    const employee1 = await createTestEmployee({ name: "Racer 1" });
+    const employee2 = await createTestEmployee({ name: "Racer 2" });
+    const modelId = await createTestModelId();
+    const assetItem = await createTestAssetItem({ modelId });
+
+    const [resultA, resultB] = await Promise.all([
+      createAssignment({ employeeId: employee1.id, assetItemId: assetItem.id }),
+      createAssignment({ employeeId: employee2.id, assetItemId: assetItem.id }),
+    ]);
+
+    const outcomes = [resultA, resultB];
+    const succeeded = outcomes.filter((r) => r.ok);
+    const failed = outcomes.filter((r) => !r.ok);
+
+    expect(succeeded).toHaveLength(1);
+    expect(failed).toHaveLength(1);
+
+    const item = await prisma.assetItem.findUniqueOrThrow({ where: { id: assetItem.id } });
+    expect(item.status).toBe("assigned");
+
+    const activeAssignments = await prisma.assignment.findMany({
+      where: { assetItemId: assetItem.id, returnedAt: null },
+    });
+    expect(activeAssignments).toHaveLength(1);
+  });
+});
+
+describe("returnAssignment", () => {
+  it("returns an AssetItem assignment: sets returnedAt, flips status back to in_stock", async () => {
+    const employee = await createTestEmployee();
+    const modelId = await createTestModelId();
+    const assetItem = await createTestAssetItem({ modelId });
+
+    const created = await createAssignment({ employeeId: employee.id, assetItemId: assetItem.id });
+    expect(created.ok).toBe(true);
+    if (!created.ok) return;
+
+    const returned = await returnAssignment(created.value.id, {});
+    expect(returned.ok).toBe(true);
+    if (!returned.ok) return;
+    expect(returned.value.returned_at).not.toBeNull();
+
+    const item = await prisma.assetItem.findUniqueOrThrow({ where: { id: assetItem.id } });
+    expect(item.status).toBe("in_stock");
+  });
+
+  it("accepts an explicit returnedAt", async () => {
+    const employee = await createTestEmployee();
+    const modelId = await createTestModelId();
+    const assetItem = await createTestAssetItem({ modelId });
+    const created = await createAssignment({ employeeId: employee.id, assetItemId: assetItem.id });
+    expect(created.ok).toBe(true);
+    if (!created.ok) return;
+
+    const explicitDate = new Date("2026-08-01T00:00:00.000Z");
+    const returned = await returnAssignment(created.value.id, { returnedAt: explicitDate });
+    expect(returned.ok).toBe(true);
+    if (!returned.ok) return;
+    expect(returned.value.returned_at).toBe(explicitDate.toISOString());
+  });
+
+  it("returns a license assignment: sets returnedAt, decrements seatsUsed", async () => {
+    const employee = await createTestEmployee();
+    const licenseId = await createTestLicenseId({ seatsTotal: 5, seatsUsed: 2 });
+
+    const created = await createAssignment({ employeeId: employee.id, licenseId });
+    expect(created.ok).toBe(true);
+    if (!created.ok) return;
+
+    let license = await prisma.license.findUniqueOrThrow({ where: { id: licenseId } });
+    expect(license.seatsUsed).toBe(3);
+
+    const returned = await returnAssignment(created.value.id, {});
+    expect(returned.ok).toBe(true);
+
+    license = await prisma.license.findUniqueOrThrow({ where: { id: licenseId } });
+    expect(license.seatsUsed).toBe(2);
+  });
+
+  it("rejects returning an already-returned assignment (double return)", async () => {
+    const employee = await createTestEmployee();
+    const modelId = await createTestModelId();
+    const assetItem = await createTestAssetItem({ modelId });
+    const created = await createAssignment({ employeeId: employee.id, assetItemId: assetItem.id });
+    expect(created.ok).toBe(true);
+    if (!created.ok) return;
+
+    const firstReturn = await returnAssignment(created.value.id, {});
+    expect(firstReturn.ok).toBe(true);
+
+    const secondReturn = await returnAssignment(created.value.id, {});
+    expect(secondReturn.ok).toBe(false);
+    if (secondReturn.ok) return;
+    expect(secondReturn.status).toBe(400);
+    expect(secondReturn.error).toMatch(/already been returned/i);
+  });
+
+  it("returns 404 when returning a nonexistent assignment", async () => {
+    const result = await returnAssignment("00000000-0000-0000-0000-000000000000", {});
+    expect(result.ok).toBe(false);
+    if (result.ok) return;
+    expect(result.status).toBe(404);
+    expect(result.error).toMatch(/not found/i);
+  });
+
+  it("closes the race: two concurrent returnAssignment calls against the same assignment — only one succeeds", async () => {
+    const employee = await createTestEmployee();
+    const licenseId = await createTestLicenseId({ seatsTotal: 5, seatsUsed: 0 });
+    const created = await createAssignment({ employeeId: employee.id, licenseId });
+    expect(created.ok).toBe(true);
+    if (!created.ok) return;
+
+    const [resultA, resultB] = await Promise.all([
+      returnAssignment(created.value.id, {}),
+      returnAssignment(created.value.id, {}),
+    ]);
+
+    const outcomes = [resultA, resultB];
+    const succeeded = outcomes.filter((r) => r.ok);
+    const failed = outcomes.filter((r) => !r.ok);
+    expect(succeeded).toHaveLength(1);
+    expect(failed).toHaveLength(1);
+
+    const license = await prisma.license.findUniqueOrThrow({ where: { id: licenseId } });
+    expect(license.seatsUsed).toBe(0);
+  });
+});
+
+describe("listAssignments", () => {
+  it("lists created assignments, newest first", async () => {
+    const employee = await createTestEmployee();
+    const modelId = await createTestModelId();
+    const item1 = await createTestAssetItem({ modelId });
+    const item2 = await createTestAssetItem({ modelId });
+
+    const first = await createAssignment({ employeeId: employee.id, assetItemId: item1.id });
+    expect(first.ok).toBe(true);
+    await new Promise((resolve) => setTimeout(resolve, 5));
+    const second = await createAssignment({ employeeId: employee.id, assetItemId: item2.id });
+    expect(second.ok).toBe(true);
+
+    const records = await listAssignments();
+    expect(records).toHaveLength(2);
+    expect(records[0].asset_item_id).toBe(item2.id);
+    expect(records[1].asset_item_id).toBe(item1.id);
+  });
+
+  it("returns an empty list when there are no assignments", async () => {
+    const records = await listAssignments();
+    expect(records).toEqual([]);
+  });
+});
+
+describe("POST /api/assignments", () => {
+  it("creates an assignment on valid input", async () => {
+    const employee = await createTestEmployee();
+    const modelId = await createTestModelId();
+    const assetItem = await createTestAssetItem({ modelId });
+
+    const res = await POST(postRequest({ employeeId: employee.id, assetItemId: assetItem.id }));
+    expect(res.status).toBe(201);
+    const body = await res.json();
+    expect(body).toMatchObject({ employee_id: employee.id, asset_item_id: assetItem.id, license_id: null });
+  });
+
+  it("rejects both assetItemId and licenseId set", async () => {
+    const employee = await createTestEmployee();
+    const modelId = await createTestModelId();
+    const assetItem = await createTestAssetItem({ modelId });
+    const licenseId = await createTestLicenseId();
+
+    const res = await POST(
+      postRequest({ employeeId: employee.id, assetItemId: assetItem.id, licenseId })
+    );
+    expect(res.status).toBe(400);
+    const body = await res.json();
+    expect(body.error).toMatch(/exactly one/i);
+  });
+
+  it("rejects neither assetItemId nor licenseId set", async () => {
+    const employee = await createTestEmployee();
+    const res = await POST(postRequest({ employeeId: employee.id }));
+    expect(res.status).toBe(400);
+    const body = await res.json();
+    expect(body.error).toMatch(/exactly one/i);
+  });
+
+  it("rejects assigning an AssetItem that isn't in_stock", async () => {
+    const employee = await createTestEmployee();
+    const modelId = await createTestModelId();
+    const assetItem = await createTestAssetItem({ modelId, status: "repair" });
+
+    const res = await POST(postRequest({ employeeId: employee.id, assetItemId: assetItem.id }));
+    expect(res.status).toBe(400);
+    const body = await res.json();
+    expect(body.error).toMatch(/not in_stock/i);
+  });
+
+  it("rejects an invalid JSON body", async () => {
+    const req = new Request("http://localhost/api/assignments", {
+      method: "POST",
+      headers: { "content-type": "application/json" },
+      body: "not json",
+    });
+    const res = await POST(req);
+    expect(res.status).toBe(400);
+    const body = await res.json();
+    expect(body.error).toMatch(/JSON/i);
+  });
+
+  it("rejects a non-object JSON body", async () => {
+    const res = await POST(postRequest("not an object"));
+    expect(res.status).toBe(400);
+    const body = await res.json();
+    expect(body.error).toMatch(/JSON object/i);
+  });
+});
+
+describe("GET /api/assignments", () => {
+  it("lists created assignments, newest first", async () => {
+    const employee = await createTestEmployee();
+    const modelId = await createTestModelId();
+    const item1 = await createTestAssetItem({ modelId });
+    const item2 = await createTestAssetItem({ modelId });
+
+    await POST(postRequest({ employeeId: employee.id, assetItemId: item1.id }));
+    await new Promise((resolve) => setTimeout(resolve, 5));
+    await POST(postRequest({ employeeId: employee.id, assetItemId: item2.id }));
+
+    const res = await GET();
+    expect(res.status).toBe(200);
+    const body = await res.json();
+    expect(body).toHaveLength(2);
+    expect(body[0].asset_item_id).toBe(item2.id);
+    expect(body[1].asset_item_id).toBe(item1.id);
+  });
+
+  it("returns an empty list when there are no assignments", async () => {
+    const res = await GET();
+    expect(res.status).toBe(200);
+    const body = await res.json();
+    expect(body).toEqual([]);
+  });
+});
+
+describe("PATCH /api/assignments/[id]", () => {
+  it("returns an assignment on valid input", async () => {
+    const employee = await createTestEmployee();
+    const modelId = await createTestModelId();
+    const assetItem = await createTestAssetItem({ modelId });
+    const createRes = await POST(postRequest({ employeeId: employee.id, assetItemId: assetItem.id }));
+    const created = await createRes.json();
+
+    const res = await PATCH(patchRequest({}), { params: { id: created.id } });
+    expect(res.status).toBe(200);
+    const body = await res.json();
+    expect(body.returned_at).not.toBeNull();
+
+    const item = await prisma.assetItem.findUniqueOrThrow({ where: { id: assetItem.id } });
+    expect(item.status).toBe("in_stock");
+  });
+
+  it("returns 404 for a nonexistent assignment id", async () => {
+    const res = await PATCH(patchRequest({}), {
+      params: { id: "00000000-0000-0000-0000-000000000000" },
+    });
+    expect(res.status).toBe(404);
+    const body = await res.json();
+    expect(body.error).toMatch(/not found/i);
+  });
+
+  it("returns 400 for a malformed id (not a UUID)", async () => {
+    const res = await PATCH(patchRequest({}), { params: { id: "not-a-uuid" } });
+    expect(res.status).toBe(400);
+    const body = await res.json();
+    expect(body.error).toMatch(/UUID/i);
+  });
+
+  it("rejects double-return with 400", async () => {
+    const employee = await createTestEmployee();
+    const modelId = await createTestModelId();
+    const assetItem = await createTestAssetItem({ modelId });
+    const createRes = await POST(postRequest({ employeeId: employee.id, assetItemId: assetItem.id }));
+    const created = await createRes.json();
+
+    await PATCH(patchRequest({}), { params: { id: created.id } });
+    const res = await PATCH(patchRequest({}), { params: { id: created.id } });
+    expect(res.status).toBe(400);
+    const body = await res.json();
+    expect(body.error).toMatch(/already been returned/i);
+  });
+
+  it("rejects an invalid JSON body", async () => {
+    const req = new Request("http://localhost/api/assignments/x", {
+      method: "PATCH",
+      headers: { "content-type": "application/json" },
+      body: "not json",
+    });
+    const res = await PATCH(req, { params: { id: "00000000-0000-0000-0000-000000000000" } });
+    expect(res.status).toBe(400);
+    const body = await res.json();
+    expect(body.error).toMatch(/JSON/i);
+  });
+
+  it("rejects a non-object JSON body", async () => {
+    const res = await PATCH(patchRequest("not an object"), {
+      params: { id: "00000000-0000-0000-0000-000000000000" },
+    });
+    expect(res.status).toBe(400);
+    const body = await res.json();
+    expect(body.error).toMatch(/JSON object/i);
+  });
+
+  it("returns 401 without a session", async () => {
+    mockedAuth.mockResolvedValue(null);
+    const res = await PATCH(patchRequest({}), {
+      params: { id: "00000000-0000-0000-0000-000000000000" },
+    });
+    expect(res.status).toBe(401);
+    const body = await res.json();
+    expect(body).toEqual({ error: "Unauthorized" });
+  });
+});
+
+describe("auth gating on /api/assignments", () => {
+  it("POST /api/assignments returns 401 without a session", async () => {
+    const employee = await createTestEmployee();
+    const modelId = await createTestModelId();
+    const assetItem = await createTestAssetItem({ modelId });
+    mockedAuth.mockResolvedValue(null);
+
+    const res = await POST(postRequest({ employeeId: employee.id, assetItemId: assetItem.id }));
+    expect(res.status).toBe(401);
+    const body = await res.json();
+    expect(body).toEqual({ error: "Unauthorized" });
+
+    const remaining = await prisma.assignment.findMany();
+    expect(remaining).toHaveLength(0);
+  });
+
+  it("GET /api/assignments returns 401 without a session", async () => {
+    mockedAuth.mockResolvedValue(null);
+    const res = await GET();
+    expect(res.status).toBe(401);
+    const body = await res.json();
+    expect(body).toEqual({ error: "Unauthorized" });
+  });
+});