- Fixed bug where using a :class:`.TypeDecorator` that implemented

a type that was also a :class:`.TypeDecorator` would fail with
Python's "Cannot create a consistent method resolution order (MRO)"
error, when any kind of SQL comparison expression were used against
an object using this type.

3 files changed, 43 insertions, 3 deletions

diff --git a/doc/build/changelog/changelog_09.rst b/doc/build/changelog/changelog_09.rst
index b2c876141..4ff73c45d 100644
--- a/doc/build/changelog/changelog_09.rst
+++ b/doc/build/changelog/changelog_09.rst
@@ -14,6 +14,17 @@
     :version: 0.9.9
 
     .. change::
+        :tags: bug, sql
+        :versions: 1.0.0
+        :tickets: 3278
+
+        Fixed bug where using a :class:`.TypeDecorator` that implemented
+        a type that was also a :class:`.TypeDecorator` would fail with
+        Python's "Cannot create a consistent method resolution order (MRO)"
+        error, when any kind of SQL comparison expression were used against
+        an object using this type.
+
+    .. change::
         :tags: bug, mysql
         :versions: 1.0.0
         :tickets: 3274
diff --git a/lib/sqlalchemy/sql/type_api.py b/lib/sqlalchemy/sql/type_api.py
index d3e0a008e..d414daf2a 100644
--- a/lib/sqlalchemy/sql/type_api.py
+++ b/lib/sqlalchemy/sql/type_api.py
@@ -630,9 +630,13 @@ class TypeDecorator(TypeEngine):
 
     @property
     def comparator_factory(self):
-        return type("TDComparator",
-                    (TypeDecorator.Comparator, self.impl.comparator_factory),
-                    {})
+        if TypeDecorator.Comparator in self.impl.comparator_factory.__mro__:
+            return self.impl.comparator_factory
+        else:
+            return type("TDComparator",
+                        (TypeDecorator.Comparator,
+                         self.impl.comparator_factory),
+                        {})
 
     def _gen_dialect_impl(self, dialect):
         """
diff --git a/test/sql/test_operators.py b/test/sql/test_operators.py
index f8ac1528f..3b8b20513 100644
--- a/test/sql/test_operators.py
+++ b/test/sql/test_operators.py
@@ -393,6 +393,31 @@ class TypeDecoratorComparatorTest(_CustomComparatorTests, fixtures.TestBase):
         return MyInteger
 
 
+class TypeDecoratorTypeDecoratorComparatorTest(
+        _CustomComparatorTests, fixtures.TestBase):
+
+    def _add_override_factory(self):
+
+        class MyIntegerOne(TypeDecorator):
+            impl = Integer
+
+            class comparator_factory(TypeDecorator.Comparator):
+
+                def __init__(self, expr):
+                    self.expr = expr
+
+                def __add__(self, other):
+                    return self.expr.op("goofy")(other)
+
+                def __and__(self, other):
+                    return self.expr.op("goofy_and")(other)
+
+        class MyIntegerTwo(TypeDecorator):
+            impl = MyIntegerOne
+
+        return MyIntegerTwo
+
+
 class TypeDecoratorWVariantComparatorTest(
         _CustomComparatorTests,
         fixtures.TestBase):