fix(media): return the exact window for non-aligned offsets

Same B9 defect as blog: Payload paginates by page, so a non-aligned
offset returned the wrong window. Fetch the straddled pages and slice
the intra-page remainder. Identical window tests now run against the
mock and the fake-payload stub.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

packages/media/src/infrastructure/repositories/media.repository.mock.test.ts

@@ -1,12 +1,48 @@
-import { describe } from "vitest";
+import { describe, expect, it } from "vitest";
 import { RecordingTracer } from "@repo/core-testing/instrumentation";
 import { MockMediaRepository } from "@/infrastructure/repositories/media.repository.mock";
 import { mediaRepositoryContract } from "@/__contracts__/media-repository.contract";
+import { mediaFactory } from "@/__factories__/media.factory";
 
 describe("MockMediaRepository", () => {
   const tracer = new RecordingTracer();
-  mediaRepositoryContract.run(
-    () => new MockMediaRepository(tracer),
-    { tracer: () => tracer },
-  );
+  mediaRepositoryContract.run(() => new MockMediaRepository(tracer), {
+    tracer: () => tracer,
+  });
+});
+
+describe("MockMediaRepository listMedia pagination windows", () => {
+  // Mirrors the window tests in media.repository.test.ts — both
+  // implementations must return identical windows for the same offsets.
+  async function buildSeededRepo() {
+    const repo = new MockMediaRepository();
+    for (let i = 1; i <= 5; i++) {
+      await repo._store(mediaFactory.build({ id: `m-${i}` }));
+    }
+    return repo;
+  }
+
+  it("returns the exact window for an aligned offset", async () => {
+    const repo = await buildSeededRepo();
+    const window = await repo.listMedia({ offset: 2, limit: 2 });
+    expect(window.map((m) => m.id)).toEqual(["m-3", "m-4"]);
+  });
+
+  it("returns the exact window for a non-aligned offset", async () => {
+    const repo = await buildSeededRepo();
+    const window = await repo.listMedia({ offset: 3, limit: 2 });
+    expect(window.map((m) => m.id)).toEqual(["m-4", "m-5"]);
+  });
+
+  it("non-aligned offset near the end returns only the remaining items", async () => {
+    const repo = await buildSeededRepo();
+    const window = await repo.listMedia({ offset: 4, limit: 3 });
+    expect(window.map((m) => m.id)).toEqual(["m-5"]);
+  });
+
+  it("offset past the end returns an empty array", async () => {
+    const repo = await buildSeededRepo();
+    const window = await repo.listMedia({ offset: 7, limit: 3 });
+    expect(window).toEqual([]);
+  });
 });

packages/media/src/infrastructure/repositories/media.repository.test.ts

@@ -12,6 +12,7 @@ function buildPayloadStub() {
   const store = new Map<string, Record<string, unknown>>();
 
   return {
+    _store: store,
     find: vi.fn(
       async ({
         limit,
@@ -31,17 +32,31 @@ function buildPayloadStub() {
       },
     ),
     findByID: vi.fn(
-      async ({ id }: { collection: string; id: string; overrideAccess?: boolean }) => {
+      async ({
+        id,
+      }: {
+        collection: string;
+        id: string;
+        overrideAccess?: boolean;
+      }) => {
         const doc = store.get(String(id));
         if (!doc) {
-          const err = Object.assign(new Error(`Not found: ${id}`), { status: 404 });
+          const err = Object.assign(new Error(`Not found: ${id}`), {
+            status: 404,
+          });
           throw err;
         }
         return doc;
       },
     ),
     delete: vi.fn(
-      async ({ id }: { collection: string; id: string; overrideAccess?: boolean }) => {
+      async ({
+        id,
+      }: {
+        collection: string;
+        id: string;
+        overrideAccess?: boolean;
+      }) => {
         store.delete(String(id));
         return { id };
       },
@@ -79,7 +94,9 @@ describe("MediaRepository", () => {
     it("returns undefined when Payload throws (not found)", async () => {
       const { getPayload } = await import("payload");
       (getPayload as ReturnType<typeof vi.fn>).mockResolvedValue({
-        findByID: vi.fn().mockRejectedValue(
+        findByID: vi
+          .fn()
+          .mockRejectedValue(
             Object.assign(new Error("Not found"), { status: 404 }),
           ),
       });
@@ -113,6 +130,51 @@ describe("MediaRepository", () => {
     });
   });
 
+  describe("listMedia pagination windows", () => {
+    // Mirrors the window tests in media.repository.mock.test.ts — both
+    // implementations must return identical windows for the same offsets.
+    async function buildSeededRepo() {
+      const stub = buildPayloadStub();
+      for (let i = 1; i <= 5; i++) {
+        stub._store.set(`m-${i}`, {
+          id: `m-${i}`,
+          alt: `Media ${i}`,
+          url: `https://cdn.example.com/${i}.png`,
+          filename: `${i}.png`,
+          mimeType: "image/png",
+          filesize: 100,
+        });
+      }
+      const { getPayload } = await import("payload");
+      (getPayload as ReturnType<typeof vi.fn>).mockResolvedValue(stub);
+      return new MediaRepository(stubPayloadConfig);
+    }
+
+    it("returns the exact window for an aligned offset", async () => {
+      const repo = await buildSeededRepo();
+      const window = await repo.listMedia({ offset: 2, limit: 2 });
+      expect(window.map((m) => m.id)).toEqual(["m-3", "m-4"]);
+    });
+
+    it("returns the exact window for a non-aligned offset", async () => {
+      const repo = await buildSeededRepo();
+      const window = await repo.listMedia({ offset: 3, limit: 2 });
+      expect(window.map((m) => m.id)).toEqual(["m-4", "m-5"]);
+    });
+
+    it("non-aligned offset near the end returns only the remaining items", async () => {
+      const repo = await buildSeededRepo();
+      const window = await repo.listMedia({ offset: 4, limit: 3 });
+      expect(window.map((m) => m.id)).toEqual(["m-5"]);
+    });
+
+    it("offset past the end returns an empty array", async () => {
+      const repo = await buildSeededRepo();
+      const window = await repo.listMedia({ offset: 7, limit: 3 });
+      expect(window).toEqual([]);
+    });
+  });
+
   describe("listMedia", () => {
     it("returns an array of mapped Media docs", async () => {
       const { getPayload } = await import("payload");

packages/media/src/infrastructure/repositories/media.repository.ts

@@ -81,14 +81,20 @@ export class MediaRepository implements IMediaRepository {
           this.logger.captureException(err, {
             tags: { feature: FEATURE, repo: REPO, method: "getMedia" },
           });
-          span.setStatus("error", err instanceof Error ? err.message : String(err));
+          span.setStatus(
+            "error",
+            err instanceof Error ? err.message : String(err),
+          );
           throw err;
         }
       },
     );
   }
 
-  async listMedia(opts?: { limit?: number; offset?: number }): Promise<Media[]> {
+  async listMedia(opts?: {
+    limit?: number;
+    offset?: number;
+  }): Promise<Media[]> {
     return this.tracer.startSpan(
       {
         name: "media.listMedia",
@@ -101,22 +107,43 @@ export class MediaRepository implements IMediaRepository {
       async (span) => {
         try {
           const payload = await getPayload({ config: this.config });
-          const result = await payload.find({
+
+          const limit = opts?.limit ?? 50;
+          const offset = opts?.offset ?? 0;
+          // Payload paginates by page, not offset. A non-aligned offset
+          // (offset % limit !== 0) straddles two pages, so fetch both and
+          // slice out the exact [offset, offset + limit) window.
+          const page = limit > 0 ? Math.floor(offset / limit) + 1 : 1;
+          const remainder = limit > 0 ? offset % limit : 0;
+
+          const findPage = (p: number) =>
+            payload.find({
               collection: "media",
-            limit: opts?.limit ?? 50,
-            page: opts?.offset
-              ? Math.floor(opts.offset / (opts.limit ?? 50)) + 1
-              : 1,
+              limit,
+              page: p,
               overrideAccess: true,
             });
-          const items = result.docs.map((d) => mapDoc(d as PayloadMediaDoc));
+
+          const first = await findPage(page);
+          let docs = first.docs;
+          if (remainder > 0) {
+            if (docs.length === limit) {
+              const second = await findPage(page + 1);
+              docs = [...docs, ...second.docs];
+            }
+            docs = docs.slice(remainder, remainder + limit);
+          }
+          const items = docs.map((d) => mapDoc(d as PayloadMediaDoc));
           span.setAttribute("count", items.length);
           return items;
         } catch (err) {
           this.logger.captureException(err, {
             tags: { feature: FEATURE, repo: REPO, method: "listMedia" },
           });
-          span.setStatus("error", err instanceof Error ? err.message : String(err));
+          span.setStatus(
+            "error",
+            err instanceof Error ? err.message : String(err),
+          );
           throw err;
         }
       },
@@ -140,7 +167,10 @@ export class MediaRepository implements IMediaRepository {
           this.logger.captureException(err, {
             tags: { feature: FEATURE, repo: REPO, method: "deleteMedia" },
           });
-          span.setStatus("error", err instanceof Error ? err.message : String(err));
+          span.setStatus(
+            "error",
+            err instanceof Error ? err.message : String(err),
+          );
           throw err;
         }
       },