strings and unicodes are compared via == rather than 'is'.

TypeDecorator uses underlying impl for mutator/comparison functions by default

lib/sqlalchemy/types.py

@@ -98,6 +98,12 @@ class TypeDecorator(AbstractType):
         return instance
     def get_dbapi_type(self, dbapi):
         return self.impl.get_dbapi_type(dbapi)
+    def copy_value(self, value):
+        return self.impl.copy_value(value)
+    def compare_values(self, x, y):
+        return self.impl.compare_values(x,y)
+    def is_mutable(self):
+        return self.impl.is_mutable()
 
 class MutableType(object):
     """a mixin that marks a Type as holding a mutable object"""
@@ -144,7 +150,6 @@ class NullTypeEngine(TypeEngine):
     def convert_result_value(self, value, dialect):
         return value
 
-    
 class String(TypeEngine):
     def __new__(cls, *args, **kwargs):
         if cls is not String or len(args) > 0 or kwargs.has_key('length'):
@@ -167,6 +172,8 @@ class String(TypeEngine):
             return value.decode(dialect.encoding)
     def get_dbapi_type(self, dbapi):
         return dbapi.STRING
+    def compare_values(self, x, y):
+        return x == y
         
 class Unicode(TypeDecorator):
     impl = String

test/orm/unitofwork.py

@@ -239,7 +239,8 @@ class MutableTypesTest(UnitOfWorkTest):
         metadata = BoundMetaData(testbase.db)
         table = Table('mutabletest', metadata,
             Column('id', Integer, primary_key=True),
-            Column('data', PickleType, nullable=False))
+            Column('data', PickleType),
+            Column('value', Unicode(30)))
         table.create()
     def tearDownAll(self):
         table.drop()
@@ -279,6 +280,22 @@ class MutableTypesTest(UnitOfWorkTest):
         print f2.data, f3.data
         assert (f3.data.x, f3.data.y) == (4,19)
         
+    def testunicode(self):
+        """test that two equivalent unicode values dont get flagged as changed.
+        
+        apparently two equal unicode objects dont compare via "is" in all cases, so this
+        tests the compare_values() call on types.String and its usage via types.Unicode."""
+        class Foo(object):pass
+        mapper(Foo, table)
+        f1 = Foo()
+        f1.value = u'hi'
+        ctx.current.flush()
+        ctx.current.clear()
+        f1 = ctx.current.get(Foo, f1.id)
+        f1.value = u'hi'
+        def go():
+            ctx.current.flush()
+        self.assert_sql_count(db, go, 0)
         
         
 class PKTest(UnitOfWorkTest):