The resolution of :class:`.ForeignKey` objects to their

target :class:`.Column` has been reworked to be as
immediate as possible, based on the moment that the
target :class:`.Column` is associated with the same
:class:`.MetaData` as this :class:`.ForeignKey`, rather
than waiting for the first time a join is constructed,
or similar. This along with other improvements allows
earlier detection of some foreign key configuration
issues.  Also included here is a rework of the
type-propagation system, so that
it should be reliable now to set the type as ``None``
on any :class:`.Column` that refers to another via
:class:`.ForeignKey` - the type will be copied from the
target column as soon as that other column is associated,
and now works for composite foreign keys as well.
[ticket:1765]

1 files changed, 3 insertions, 1 deletions

diff --git a/lib/sqlalchemy/sql/compiler.py b/lib/sqlalchemy/sql/compiler.py
index dd2a6e08c..f1fe53b73 100644
--- a/lib/sqlalchemy/sql/compiler.py
+++ b/lib/sqlalchemy/sql/compiler.py
@@ -2415,7 +2415,9 @@ class GenericTypeCompiler(engine.TypeCompiler):
         return self.visit_VARCHAR(type_)
 
     def visit_null(self, type_):
-        raise NotImplementedError("Can't generate DDL for the null type")
+        raise exc.CompileError("Can't generate DDL for %r; "
+                            "did you forget to specify a "
+                            "type on this Column?" % type_)
 
     def visit_type_decorator(self, type_):
         return self.process(type_.type_engine(self.dialect))