Added a test for the change of the condition in DeReference processing

This checks DBRef conversion using DynamicField with the ordering
guarantee.

tests/test_dereference.py

@@ -2,10 +2,15 @@
 import unittest
 
 from bson import DBRef, ObjectId
+from collections import OrderedDict
 
 from mongoengine import *
 from mongoengine.connection import get_db
 from mongoengine.context_managers import query_counter
+from mongoengine.python_support import IS_PYMONGO_3
+from mongoengine.base import TopLevelDocumentMetaclass
+if IS_PYMONGO_3:
+    from bson import CodecOptions
 
 
 class FieldTest(unittest.TestCase):
@@ -1287,5 +1292,70 @@ class FieldTest(unittest.TestCase):
 
             self.assertEqual(q, 2)
 
+    def test_dynamic_field_dereference(self):
+        class Merchandise(Document):
+            name = StringField()
+            price = IntField()
+
+        class Store(Document):
+            merchandises = DynamicField()
+
+        Merchandise.drop_collection()
+        Store.drop_collection()
+
+        merchandises = {
+            '#1': Merchandise(name='foo', price=100).save(),
+            '#2': Merchandise(name='bar', price=120).save(),
+            '#3': Merchandise(name='baz', price=110).save(),
+        }
+        Store(merchandises=merchandises).save()
+
+        store = Store.objects().first()
+        for obj in store.merchandises.values():
+            self.assertFalse(isinstance(obj, Merchandise))
+
+        store.select_related()
+        for obj in store.merchandises.values():
+            self.assertTrue(isinstance(obj, Merchandise))
+
+    def test_dynamic_field_dereference_with_ordering_guarantee_on_pymongo3(self):
+        # This is because 'codec_options' is supported on pymongo3 or later
+        if IS_PYMONGO_3:
+            class OrderedDocument(Document):
+                my_metaclass = TopLevelDocumentMetaclass
+                __metaclass__ = TopLevelDocumentMetaclass
+
+                @classmethod
+                def _get_collection(cls):
+                    collection = super(OrderedDocument, cls)._get_collection()
+                    opts = CodecOptions(document_class=OrderedDict)
+
+                    return collection.with_options(codec_options=opts)
+
+            class Merchandise(Document):
+                name = StringField()
+                price = IntField()
+
+            class Store(OrderedDocument):
+                merchandises = DynamicField(container_class=OrderedDict)
+
+            Merchandise.drop_collection()
+            Store.drop_collection()
+
+            merchandises = OrderedDict()
+            merchandises['#1'] = Merchandise(name='foo', price=100).save()
+            merchandises['#2'] = Merchandise(name='bar', price=120).save()
+            merchandises['#3'] = Merchandise(name='baz', price=110).save()
+
+            Store(merchandises=merchandises).save()
+
+            store = Store.objects().first()
+
+            store.select_related()
+
+            # confirms that the load data order is same with the one at storing
+            self.assertTrue(type(store.merchandises), OrderedDict)
+            self.assertEqual(','.join(store.merchandises.keys()), '#1,#2,#3')
+
 if __name__ == '__main__':
     unittest.main()