test: add test case for #15138

test/document.test.js

@@ -3090,6 +3090,51 @@ describe('document', function() {
       assert.strictEqual(nestedModel.isNew, false);
     });
 
+    it('manual populattion with ref function (gh-15138)', async function() {
+      const userSchema = new mongoose.Schema({
+        username: { type: String },
+        phone: { type: mongoose.Schema.Types.ObjectId, ref: 'phones' }
+      });
+
+      const userSchemaWithFunc = new mongoose.Schema({
+        username: { type: String },
+        phone: {
+          type: mongoose.Schema.Types.ObjectId,
+          ref: function() {
+            return 'phones';
+          }
+        }
+      });
+
+      const phoneSchema = new mongoose.Schema({
+        phoneNumber: { type: String },
+        extension: { type: String }
+      });
+
+      const User = db.model('users', userSchema);
+      const UserWithFunc = db.model('usersWithFunc', userSchemaWithFunc);
+      const Phone = db.model('phones', phoneSchema);
+
+      const phone = await Phone.create({
+        phoneNumber: '123456789',
+        extension: '123'
+      });
+
+      const user = await User.create({
+        username: 'John Doe',
+        phone
+      });
+
+      const userWithFunc = await UserWithFunc.create({
+        username: 'John Doe',
+        phone
+      });
+
+      assert.ok(user.populated('phone'));
+      assert.ok(userWithFunc.populated('phone'));
+      assert.equal(userWithFunc.phone.extension, '123');
+    });
+
     it('manual population with refPath (gh-7070)', async function() {
       const ChildModelSchema = new mongoose.Schema({
         name: String
@@ -14190,4 +14235,3 @@ describe('Check if instance function that is supplied in schema option is availa
     assert.equal(TestDocument.instanceFn(), 'Returned from DocumentInstanceFn');
   });
 });
-