use property name rather than db column name for form_choices

flask_admin/contrib/sqla/form.py

@@ -243,7 +243,7 @@ class AdminModelConverter(ModelConverterBase):
             form_choices = getattr(self.view, 'form_choices', None)
 
             if mapper.class_ == self.view.model and form_choices:
-                choices = form_choices.get(column.key)
+                choices = form_choices.get(prop.key)
                 if choices:
                     return form.Select2Field(
                         choices=choices,