Add support for mocking MongoEngine based on mongomock

Using `mongomock://` scheme in URI enables the mocking. Fix #1045.

mongoengine/connection.py

@@ -54,8 +54,11 @@ def register_connection(alias, name=None, host=None, port=None,
     }
 
     # Handle uri style connections
-    if "://" in conn_settings['host']:
-        uri_dict = uri_parser.parse_uri(conn_settings['host'])
+    conn_host = conn_settings['host']
+    if conn_host.startswith('mongomock://'):
+        conn_settings['is_mock'] = True
+    elif '://' in conn_host:
+        uri_dict = uri_parser.parse_uri(conn_host)
         conn_settings.update({
             'name': uri_dict.get('database') or name,
             'username': uri_dict.get('username'),
@@ -106,7 +109,19 @@ def get_connection(alias=DEFAULT_CONNECTION_NAME, reconnect=False):
         conn_settings.pop('password', None)
         conn_settings.pop('authentication_source', None)
 
-        connection_class = MongoClient
+        is_mock = conn_settings.pop('is_mock', None)
+        if is_mock:
+            # Use MongoClient from mongomock
+            try:
+                import mongomock
+            except ImportError:
+                raise RuntimeError('You need mongomock installed '
+                                   'to mock MongoEngine.')
+            connection_class = mongomock.MongoClient
+        else:
+            # Use MongoClient from pymongo
+            connection_class = MongoClient
+
         if 'replicaSet' in conn_settings:
             # Discard port since it can't be used on MongoReplicaSetClient
             conn_settings.pop('port', None)