start to work on error messages, allow foreign_keys as only argument

if otherwise can't determine join condition due to no fks

1 files changed, 7 insertions, 0 deletions

diff --git a/lib/sqlalchemy/exc.py b/lib/sqlalchemy/exc.py
index 91ffc2811..f28bd8a07 100644
--- a/lib/sqlalchemy/exc.py
+++ b/lib/sqlalchemy/exc.py
@@ -25,6 +25,13 @@ class ArgumentError(SQLAlchemyError):
 
     """
 
+class NoForeignKeysError(ArgumentError):
+    """Raised when no foreign keys can be located between two selectables
+    during a join."""
+
+class AmbiguousForeignKeysError(ArgumentError):
+    """Raised when more than one foreign key matching can be located
+    between two selectables during a join."""
 
 class CircularDependencyError(SQLAlchemyError):
     """Raised by topological sorts when a circular dependency is detected.