Compare to metadata_impl in compare_type() to guard against custom TypeDecorator

Fixed bug where usage of a custom TypeDecorator which returns a
per-dialect type via :meth:`.TypeDecorator.load_dialect_impl` that differs
significantly from the default "impl" for the type decorator would fail
to compare correctly during autogenerate.

Change-Id: I384df35be9513bf8a2ae55e7daa9a52c23108a49
Fixes: #395

1 files changed, 22 insertions, 1 deletions

diff --git a/tests/test_autogen_diffs.py b/tests/test_autogen_diffs.py
index 04c9e96..4816134 100644
--- a/tests/test_autogen_diffs.py
+++ b/tests/test_autogen_diffs.py
@@ -5,7 +5,8 @@ from sqlalchemy import MetaData, Column, Table, Integer, String, Text, \
     TypeDecorator, CheckConstraint, text, PrimaryKeyConstraint, \
     ForeignKeyConstraint, VARCHAR, DECIMAL, DateTime, BigInteger, BIGINT, \
     SmallInteger
-from sqlalchemy.types import NULLTYPE
+from sqlalchemy.dialects import sqlite
+from sqlalchemy.types import NULLTYPE, VARBINARY
 from sqlalchemy.engine.reflection import Inspector
 
 from alembic.operations import ops
@@ -592,6 +593,26 @@ class CompareTypeSpecificityTest(TestBase):
         return impl.DefaultImpl(
             default.DefaultDialect(), None, False, True, None, {})
 
+    def test_typedec_to_nonstandard(self):
+
+        class PasswordType(TypeDecorator):
+            impl = VARBINARY
+
+            def copy(self, **kw):
+                return PasswordType(self.impl.length)
+
+            def load_dialect_impl(self, dialect):
+                if dialect.name == 'default':
+                    impl = sqlite.NUMERIC(self.length)
+                else:
+                    impl = VARBINARY(self.length)
+                return dialect.type_descriptor(impl)
+
+        impl = self._fixture()
+        impl.compare_type(
+            Column('x', sqlite.NUMERIC(50)),
+            Column('x', PasswordType(50)))
+
     def test_string(self):
         t1 = String(30)
         t2 = String(40)