Updates must have an operation

Closes #387

docs/changelog.rst

@@ -5,6 +5,7 @@ Changelog
 Changes in dev
 ==============
 
+- Error raised if update doesn't have an operation
 - DeReferencing is now thread safe
 - Errors raised if trying to perform a join in a query
 - Updates can now take __raw__ queries

mongoengine/queryset.py

@@ -1332,11 +1332,14 @@ class QuerySet(object):
 
             key = '.'.join(parts)
 
+            if not op:
+                raise InvalidQueryError("Updates must supply an operation eg: set__FIELD=value")
+
             if op:
                 value = {key: value}
                 key = '$' + op
 
-            if op is None or key not in mongo_update:
+            if key not in mongo_update:
                 mongo_update[key] = value
             elif key in mongo_update and isinstance(mongo_update[key], dict):
                 mongo_update[key].update(value)

tests/document.py

@@ -1432,6 +1432,12 @@ class DocumentTest(unittest.TestCase):
 
         self.assertRaises(OperationError, update_no_value_raises)
 
+        def update_no_op_raises():
+            person = self.Person.objects.first()
+            person.update(name="Dan")
+
+        self.assertRaises(InvalidQueryError, update_no_op_raises)
+
     def test_embedded_update(self):
         """
         Test update on `EmbeddedDocumentField` fields