diff --git a/mongoengine/queryset/base.py b/mongoengine/queryset/base.py
index 862a1826..5f11c01e 100644
--- a/mongoengine/queryset/base.py
+++ b/mongoengine/queryset/base.py
@@ -1845,10 +1845,7 @@ class BaseQuerySet(object):
         # remove it from the doc (we always fetch it so that we can properly
         # construct documents).
         fields = self._loaded_fields
-        if fields and '_id' in doc and (
-            (fields.value == QueryFieldList.ONLY and '_id' not in fields.fields) or
-            (fields.value == QueryFieldList.EXCLUDE and '_id' in fields.fields)
-        ):
+        if fields and '_id' in doc and fields.value == QueryFieldList.EXCLUDE and '_id' in fields.fields:
             del doc['_id']
 
         return doc
diff --git a/tests/queryset/queryset.py b/tests/queryset/queryset.py
index b44c36df..586913aa 100644
--- a/tests/queryset/queryset.py
+++ b/tests/queryset/queryset.py
@@ -4635,7 +4635,7 @@ class QuerySetTest(unittest.TestCase):
             ip = StringField()
 
         class User(Document):
-            id = ObjectIdField('_id')
+            id = StringField(primary_key=True)
             name = StringField()
             age = IntField()
             price = DecimalField()
@@ -4643,9 +4643,10 @@ class QuerySetTest(unittest.TestCase):
 
         User.drop_collection()
 
-        User.objects.create(name="Bob Dole", age=89, price=Decimal('1.11'))
+        User.objects.create(id='Bob', name="Bob Dole", age=89, price=Decimal('1.11'))
         User.objects.create(
-            name="Barack Obama",
+            id='Barak',
+            name="Barak Obama",
             age=51,
             price=Decimal('2.22'),
             last_login=LastLogin(
@@ -4673,7 +4674,7 @@ class QuerySetTest(unittest.TestCase):
         self.assertIsInstance(results[1], dict)
         self.assertEqual(results[0]['name'], 'Bob Dole')
         self.assertEqual(results[0]['price'], 1.11)
-        self.assertEqual(results[1]['name'], 'Barack Obama')
+        self.assertEqual(results[1]['name'], 'Barak Obama')
         self.assertEqual(results[1]['price'], 2.22)
 
         users = User.objects.only('name', 'last_login').as_pymongo()
@@ -4681,10 +4682,12 @@ class QuerySetTest(unittest.TestCase):
         self.assertIsInstance(results[0], dict)
         self.assertIsInstance(results[1], dict)
         self.assertEqual(results[0], {
+            '_id': 'Bob',
             'name': 'Bob Dole'
         })
         self.assertEqual(results[1], {
-            'name': 'Barack Obama',
+            '_id': 'Barak',
+            'name': 'Barak Obama',
             'last_login': {
                 'location': 'White House',
                 'ip': '104.107.108.116'
@@ -4701,8 +4704,8 @@ class QuerySetTest(unittest.TestCase):
                 db_field='password_salt', required=True)
 
         User.drop_collection()
-        User(email="ross@example.com", password_salt="SomeSalt",
-             password_hash="SomeHash").save()
+        user = User(email="ross@example.com", password_salt="SomeSalt",
+                    password_hash="SomeHash").save()
 
         serialized_user = User.objects.exclude(
             'password_salt', 'password_hash').as_pymongo()[0]
@@ -4714,11 +4717,16 @@ class QuerySetTest(unittest.TestCase):
 
         serialized_user = User.objects.exclude(
             'password_salt').only('email').as_pymongo()[0]
+        self.assertEqual(set(['_id', 'email']), set(serialized_user.keys()))
+
+        serialized_user = User.objects.exclude(
+            'password_salt', 'id').only('email').as_pymongo()[0]
         self.assertEqual(set(['email']), set(serialized_user.keys()))
 
         serialized_user = User.objects.exclude(
             'password_salt').only('email').to_json()
-        self.assertEqual('[{"email": "ross@example.com"}]', serialized_user)
+        self.assertEqual('[{"_id": {"$oid": "%s"}, "email": "ross@example.com"}]' % user.id,
+                         serialized_user)
 
     def test_only_after_count(self):
         """Test that only() works after count()"""