Fix: Handle collection.objects when it's a partial function

mongoengine/dereference.py

@@ -1,3 +1,5 @@
+from functools import partial
+
 from bson import SON, DBRef
 
 from mongoengine.base import (
@@ -174,7 +176,10 @@ def _fetch_objects(self, doc_type=None):
                 refs = [
                     dbref for dbref in dbrefs if (col_name, dbref) not in object_map
                 ]
-                references = collection.objects.in_bulk(refs)
+                if isinstance(collection.objects, partial):
+                    references = collection.objects().in_bulk(refs)
+                else:
+                    references = collection.objects.in_bulk(refs)
                 for key, doc in references.items():
                     object_map[(col_name, key)] = doc
             else:  # Generic reference: use the refs data to convert to document

tests/queryset/test_queryset.py

@@ -4016,6 +4016,28 @@ def objects(klass, queryset):
         assert 0 == Foo.objects.count()
         assert 1 == Bar.objects.count()
 
+    def test_select_related_when_referenced_has_custom_queryset_manager(self):
+        class Foo(Document):
+            @queryset_manager
+            def objects(klass, queryset, arg1=None, arg2=None, **kwargs):
+                return queryset(**kwargs)
+
+        class Bar(Document):
+            foo = ReferenceField(Foo)
+
+        Foo.drop_collection()
+        Bar.drop_collection()
+
+        foo = Foo()
+        foo.save()
+
+        Bar(foo=foo).save()
+
+        Bar.objects().select_related()
+
+        assert 1 == Foo.objects().count()
+        assert 1 == Bar.objects().count()
+
     def test_query_value_conversion(self):
         """Ensure that query values are properly converted when necessary."""