diff --git a/packages/data-sources/file-jsonl/src/index.ts b/packages/data-sources/file-jsonl/src/index.ts
index c39170a..433ffb8 100644
--- a/packages/data-sources/file-jsonl/src/index.ts
+++ b/packages/data-sources/file-jsonl/src/index.ts
@@ -148,8 +148,8 @@ export class JsonLinesDataSource<
     const id = idConfig.deserialize(idSerialized);
     const { [idAttr]: idFromResource, ...data } = (dataWithId as any);
     const dataToEmplace = {
-      ...data,
       [idAttr]: id,
+      ...data,
     } as Data;
 
     if (existing) {
diff --git a/packages/data-sources/file-jsonl/test/index.test.ts b/packages/data-sources/file-jsonl/test/index.test.ts
index 185c9f0..1b3c126 100644
--- a/packages/data-sources/file-jsonl/test/index.test.ts
+++ b/packages/data-sources/file-jsonl/test/index.test.ts
@@ -6,6 +6,10 @@ import {DataSource} from '@modal-sh/yasumi/dist/types/backend';
 
 vi.mock('fs/promises');
 
+const toJsonl = (dummyItems: unknown[]) => dummyItems.map((i) => JSON.stringify(i)).join('\n');
+
+const ID_ATTR = 'id' as const;
+
 describe('prepareResource', () => {
   beforeAll(() => {
     const mockWriteFile = writeFile as Mock;
@@ -20,8 +24,6 @@ describe('prepareResource', () => {
   });
 });
 
-const toJsonl = (dummyItems: unknown[]) => dummyItems.map((i) => JSON.stringify(i)).join('\n')
-
 describe('methods', () => {
   const dummyItems = [
     {
@@ -45,7 +47,7 @@ describe('methods', () => {
   beforeEach(() => {
     mockGenerationStrategy = vi.fn();
     const r = resource(schema)
-      .id('id', {
+      .id(ID_ATTR, {
         generationStrategy: mockGenerationStrategy,
         schema: v.any(),
         serialize: (id) => id.toString(),
@@ -100,8 +102,9 @@ describe('methods', () => {
 
     describe('getById', () => {
       it('works', async () => {
-        const item = await ds.getById('2'); // ID is always a string because it originates from URLs
-        const expected = dummyItems.find((i) => i.id === 2);
+        const id = 2;
+        const item = await ds.getById(id.toString()); // ID is always a string because it originates from URLs
+        const expected = dummyItems.find((i) => i[ID_ATTR] === id);
         expect(item).toEqual(expected);
       });
     });
@@ -142,15 +145,15 @@ describe('methods', () => {
         await ds.delete('1');
         expect(mockWriteFile).toBeCalledWith(
           expect.any(String),
-          toJsonl(dummyItems.filter((d) => d.id !== 1)),
+          toJsonl(dummyItems.filter((d) => d[ID_ATTR] !== 1)),
         );
       });
     });
 
     describe('emplace', () => {
-      it('works', async () => {
+      it('replaces existing data', async () => {
         const data = {
-          id: 2,
+          [ID_ATTR]: 2,
           name: 'foo',
         };
         const { id, ...etcData } = data;
@@ -162,20 +165,41 @@ describe('methods', () => {
         expect(mockWriteFile).toBeCalledWith(
           expect.any(String),
           toJsonl(dummyItems.map((d) =>
-            d.id === 2
-              // ID will be defined last, since we are just writing to file, we need strict ordering
-              ? { ...etcData, id }
+            d[ID_ATTR] === data[ID_ATTR]
+              // ID will be defined first, since we are just writing to file, we need strict ordering
+              ? { [ID_ATTR]: id, ...etcData }
               : d
           )),
         );
         expect(newItem).toEqual([data, false]);
       });
+
+      it('creates new data', async () => {
+        const data = {
+          [ID_ATTR]: 4,
+          name: 'quux',
+        };
+        const { [ID_ATTR]: id, ...etcData } = data;
+        const newItem = await ds.emplace(
+          id.toString(),
+          etcData,
+        );
+
+        expect(mockWriteFile).toBeCalledWith(
+          expect.any(String),
+          toJsonl([
+            ...dummyItems,
+            data
+          ]),
+        );
+        expect(newItem).toEqual([data, true]);
+      });
     });
 
     describe('patch', () => {
       it('works', async () => {
         const data = {
-          id: 2,
+          [ID_ATTR]: 2,
           name: 'foo',
         };
         const { id, ...etcData } = data;
@@ -187,8 +211,9 @@ describe('methods', () => {
         expect(mockWriteFile).toBeCalledWith(
           expect.any(String),
           toJsonl(dummyItems.map((d) =>
-            d.id === 2
-              ? { ...etcData, id }
+            d[ID_ATTR] === data[ID_ATTR]
+              // ID will be defined first, since we are just writing to file, we need strict ordering
+              ? { [ID_ATTR]: id, ...etcData }
               : d
           )),
         );
@@ -198,7 +223,7 @@ describe('methods', () => {
 
     describe('newId', () => {
       it('works', async () => {
-        const v = 'newId';
+        const v = Math.floor(Math.random() * Number.MAX_SAFE_INTEGER);
         mockGenerationStrategy.mockResolvedValueOnce(v);
         const id = await ds.newId();
         expect(id).toBe(v);