fix change tracking for ComplexBaseFields

mongoengine/base/fields.py

@@ -205,12 +205,6 @@ class ComplexBaseField(BaseField):
 
         return value
 
-    def __set__(self, instance, value):
-        """Descriptor for assigning a value to a field in a document.
-        """
-        instance._data[self.name] = value
-        instance._mark_as_changed(self.name)
-
     def to_python(self, value):
         """Convert a MongoDB-compatible type to a Python type.
         """

tests/fields/fields.py

@@ -808,6 +808,27 @@ class FieldTest(unittest.TestCase):
 
         self.assertRaises(ValidationError, e.save)
 
+    def test_complex_field_same_value_not_changed(self):
+        """
+        If a complex field is set to the same value, it should not be marked as
+        changed.
+        """
+        class Simple(Document):
+            mapping = ListField()
+
+        Simple.drop_collection()
+        e = Simple().save()
+        e.mapping = []
+        self.assertEqual([], e._changed_fields)
+
+        class Simple(Document):
+            mapping = DictField()
+
+        Simple.drop_collection()
+        e = Simple().save()
+        e.mapping = {}
+        self.assertEqual([], e._changed_fields)
+
     def test_list_field_complex(self):
         """Ensure that the list fields can handle the complex types."""