2 files changed, 56 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
diff --git a/test/ext/test_mutable.py b/test/ext/test_mutable.py
index dc0b5ba1c..305eb8c3a 100644
--- a/test/ext/test_mutable.py
+++ b/test/ext/test_mutable.py
@@ -332,6 +332,59 @@ class MutableAssociationScalarJSONTest(_MutableDictTestBase, fixtures.MappedTest
         )
 
 
+class CustomMutableAssociationScalarJSONTest(_MutableDictTestBase, fixtures.MappedTest):
+
+    CustomMutableDict = None
+
+    @classmethod
+    def _type_fixture(cls):
+        if not(getattr(cls, 'CustomMutableDict')):
+            MutableDict = super(CustomMutableAssociationScalarJSONTest, cls)._type_fixture()
+            class CustomMutableDict(MutableDict):
+                pass
+            cls.CustomMutableDict = CustomMutableDict
+        return cls.CustomMutableDict
+
+    @classmethod
+    def define_tables(cls, metadata):
+        import json
+
+        class JSONEncodedDict(TypeDecorator):
+            impl = VARCHAR(50)
+
+            def process_bind_param(self, value, dialect):
+                if value is not None:
+                    value = json.dumps(value)
+
+                return value
+
+            def process_result_value(self, value, dialect):
+                if value is not None:
+                    value = json.loads(value)
+                return value
+
+        CustomMutableDict = cls._type_fixture()
+        CustomMutableDict.associate_with(JSONEncodedDict)
+
+        Table('foo', metadata,
+            Column('id', Integer, primary_key=True,
+                            test_needs_autoincrement=True),
+            Column('data', JSONEncodedDict),
+            Column('unrelated_data', String(50))
+        )
+
+    def test_pickle_parent(self):
+        # Picklers don't know how to pickle CustomMutableDict, but we aren't testing that here
+        pass
+
+    def test_coerce(self):
+        sess = Session()
+        f1 = Foo(data={'a': 'b'})
+        sess.add(f1)
+        sess.flush()
+        eq_(type(f1.data), self._type_fixture())
+
+
 class _CompositeTestBase(object):
     @classmethod
     def define_tables(cls, metadata):