Raise at Core / ORM concrete inh level for label overlap

Fixed regression where the :class:`.ConcreteBase` would fail to map at all
when a mapped column name overlapped with the discriminator column name,
producing an assertion error. The use case here did not function correctly
in 1.3 as the polymorphic union would produce a query that ignored the
discriminator column entirely, while emitting duplicate column warnings. As
1.4's architecture cannot easily reproduce this essentially broken behavior
of 1.3 at the ``select()`` level right now, the use case now raises an
informative error message instructing the user to use the
``.ConcreteBase._concrete_discriminator_name`` attribute to resolve the
conflict. To assist with this configuration,
``.ConcreteBase._concrete_discriminator_name`` may be placed on the base
class only where it will be automatically used by subclasses; previously
this was not the case.

Fixes: #6090
Change-Id: I8b7d01e4c9ea0dc97f30b8cd658b3505b24312a7

1 files changed, 15 insertions, 6 deletions

diff --git a/lib/sqlalchemy/sql/elements.py b/lib/sqlalchemy/sql/elements.py
index 29023c9fe..26c03b57b 100644
--- a/lib/sqlalchemy/sql/elements.py
+++ b/lib/sqlalchemy/sql/elements.py
@@ -4330,12 +4330,21 @@ class Label(roles.LabeledColumnExprRole, ColumnElement):
             disallow_is_literal=True,
             name_is_truncatable=isinstance(name, _truncated_label),
         )
-        # TODO: want to remove this assertion at some point.  all
-        # _make_proxy() implementations will give us back the key that
-        # is our "name" in the first place.  based on this we can
-        # safely return our "self.key" as the key here, to support a new
-        # case where the key and name are separate.
-        assert key == self.name
+
+        # there was a note here to remove this assertion, which was here
+        # to determine if we later could support a use case where
+        # the key and name of a label are separate.  But I don't know what
+        # that case was.  For now, this is an unexpected case that occurs
+        # when a label name conflicts with other columns and select()
+        # is attempting to disambiguate an explicit label, which is not what
+        # the user would want.   See issue #6090.
+        if key != self.name:
+            raise exc.InvalidRequestError(
+                "Label name %s is being renamed to an anonymous label due "
+                "to disambiguation "
+                "which is not supported right now.  Please use unique names "
+                "for explicit labels." % (self.name)
+            )
 
         e._propagate_attrs = selectable._propagate_attrs
         e._proxies.append(self)