FormAlchemy with model inheritance

diff.patch

diff --git a/formalchemy/tests/__init__.py b/formalchemy/tests/__init__.py
index 7dcacd9..c9c9d6c 100644
--- a/formalchemy/tests/__init__.py
+++ b/formalchemy/tests/__init__.py
@@ -80,6 +80,18 @@ class One(Base):
     __tablename__ = 'ones'
     id = Column(Integer, primary_key=True)
 
+class Root(Base):
+    __tablename__ = 'root'
+    id = Column(Integer, primary_key=True)
+    foo = Column('type', String(50))
+    __mapper_args__ = {'polymorphic_on': foo}
+
+class Ext(Root):
+    __tablename__ = 'ext'
+    __mapper_args__ = {'polymorphic_identity': 'ext'}
+    id = Column(Integer, ForeignKey('root.id'), primary_key=True)
+    bar = Column(String(50))
+
 class Two(Base):
     __tablename__ = 'twos'
     id = Column(Integer, primary_key=True)
diff --git a/formalchemy/tests/test_inheritance.py b/formalchemy/tests/test_inheritance.py
new file mode 100644
index 0000000..fa0e7d3
--- /dev/null
+++ b/formalchemy/tests/test_inheritance.py
@@ -0,0 +1,15 @@
+__doc__ = r"""
+>>> from formalchemy.tests import *
+
+>>> FieldSet.default_renderers = original_renderers.copy()
+>>> hasattr(Ext, 'foo')
+True
+>>> fs = FieldSet(Ext)
+>>> fs
+<FieldSet with ['id', 'foo', 'bar']>
+
+"""
+
+if __name__ == '__main__':
+    import doctest
+    doctest.testmod()

results.txt

Failed example:
    fs
Expected:
    <FieldSet with ['id', 'foo', 'bar']>
Got:
    <FieldSet with ['id', 'bar']>