test(document): add repro for #4542

test/document.test.js

@@ -3226,6 +3226,34 @@ describe('document', function() {
       });
     });
 
+    it('composite _ids (gh-4542)', function(done) {
+      var schema = new Schema({
+        _id: {
+          key1: String,
+          key2: String
+        },
+        content: String
+      }, { retainKeyOrder: true });
+
+      var Model = db.model('gh4542', schema);
+
+      var object = new Model();
+      object._id = {key1: 'foo', key2: 'bar'};
+      object.save().
+        then(function(obj) {
+          obj.content = 'Hello';
+          return obj.save();
+        }).
+        then(function(obj) {
+          return Model.findOne({ _id: obj._id });
+        }).
+        then(function(obj) {
+          assert.equal(obj.content, 'Hello');
+          done();
+        }).
+        catch(done);
+    });
+
     it('setting full path under single nested schema works (gh-4578) (gh-4528)', function(done) {
       var ChildSchema = new mongoose.Schema({
         age: Number