- added NoReferencedColumnError, common base class of NoReferenceError

- relation() won't hide unrelated ForeignKey errors inside of
the "please specify primaryjoin" message when determining
join condition.

1 files changed, 7 insertions, 1 deletions

diff --git a/lib/sqlalchemy/exc.py b/lib/sqlalchemy/exc.py
index 96c2e4177..f82c632f6 100644
--- a/lib/sqlalchemy/exc.py
+++ b/lib/sqlalchemy/exc.py
@@ -63,9 +63,15 @@ class InvalidRequestError(SQLAlchemyError):
 class NoSuchColumnError(KeyError, InvalidRequestError):
     """A nonexistent column is requested from a ``RowProxy``."""
 
-class NoReferencedTableError(InvalidRequestError):
+class NoReferenceError(InvalidRequestError):
+    """Raised by ``ForeignKey`` to indicate a reference cannot be resolved."""
+    
+class NoReferencedTableError(NoReferenceError):
     """Raised by ``ForeignKey`` when the referred ``Table`` cannot be located."""
 
+class NoReferencedColumnError(NoReferenceError):
+    """Raised by ``ForeignKey`` when the referred ``Column`` cannot be located."""
+
 class NoSuchTableError(InvalidRequestError):
     """Table does not exist or is not visible to a connection."""