feat: add objectIdEqualityTester helper

README.md

@@ -129,3 +129,17 @@ test("my test", async ({ db, mongoClient }) => {
   expect(await users.countDocuments()).toBe(1);
 });
 ```
+
+### Checking object id equality
+
+If you encounter the error "Compared values have no visual difference." when comparing object ids, you'll likely need to add a [custom equality tester](https://vitest.dev/api/expect.html#expect-addequalitytesters) to vitest. This module exports a setup file for this purpose:
+
+```js
+import { defineConfig } from "vitest/config";
+
+export default defineConfig({
+  test: {
+    setupFiles: ["vitest-mms/objectIdEqualityTester"],
+  },
+});
+```

package.json

@@ -4,6 +4,11 @@
   "version": "0.1.5",
   "description": "mongodb-memory-server integration for vitest",
   "exports": {
+    "./objectIdEqualityTester": {
+      "types": "./dist/objectIdEqualityTester.d.ts",
+      "require": "./dist/objectIdEqualityTester.cjs",
+      "import": "./dist/objectIdEqualityTester.mjs"
+    },
     "./setupFile": {
       "types": "./dist/setupFile.d.ts",
       "require": "./dist/setupFile.cjs",

src/objectIdEqualityTester.ts

@@ -0,0 +1,9 @@
+import { ObjectId } from "mongodb";
+import { expect } from "vitest";
+
+expect.addEqualityTesters([
+  // https://vitest.dev/api/expect.html#expect-addequalitytesters
+  function objectIdEqualityTester(a: unknown, b: unknown) {
+    if (a instanceof ObjectId && b instanceof ObjectId) return a.equals(b);
+  },
+]);