fix MutableDict.coerce

If a class inherited from MutableDict (say, for instance, to add an update() method), coerce() would give back an instance of MutableDict instead of an instance of the derived class.

1 files changed, 3 insertions, 3 deletions

diff --git a/lib/sqlalchemy/ext/mutable.py b/lib/sqlalchemy/ext/mutable.py
index 7469bcbda..1a4568f23 100644
--- a/lib/sqlalchemy/ext/mutable.py
+++ b/lib/sqlalchemy/ext/mutable.py
@@ -627,10 +627,10 @@ class MutableDict(Mutable, dict):
 
     @classmethod
     def coerce(cls, key, value):
-        """Convert plain dictionary to MutableDict."""
-        if not isinstance(value, MutableDict):
+        """Convert plain dictionary to instance of this class."""
+        if not isinstance(value, cls):
             if isinstance(value, dict):
-                return MutableDict(value)
+                return cls(value)
             return Mutable.coerce(key, value)
         else:
             return value