Add informative failure modes to _DeferredMapperConfig

Added some helper exceptions that invoke when a mapping based on
:class:`.AbstractConcreteBase`, :class:`.DeferredReflection`, or
:class:`.AutoMap` is used before the mapping is ready to be used, which
contain descriptive information on the class, rather than falling through
into other failure modes that are less informative.

Fixes: #4470
Change-Id: I9bc51697f63cedaa7809a0adb17b2398c209e289

1 files changed, 12 insertions, 0 deletions

diff --git a/lib/sqlalchemy/ext/declarative/base.py b/lib/sqlalchemy/ext/declarative/base.py
index 80cd23bc8..62db282d1 100644
--- a/lib/sqlalchemy/ext/declarative/base.py
+++ b/lib/sqlalchemy/ext/declarative/base.py
@@ -15,6 +15,7 @@ from ... import event
 from ... import exc
 from ... import util
 from ...orm import class_mapper
+from ...orm import exc as orm_exc
 from ...orm import mapper
 from ...orm import synonym
 from ...orm.attributes import QueryableAttribute
@@ -717,6 +718,17 @@ class _DeferredMapperConfig(_MapperConfig):
         return isinstance(class_, type) and weakref.ref(class_) in cls._configs
 
     @classmethod
+    def raise_unmapped_for_cls(cls, class_):
+        if hasattr(class_, "_sa_raise_deferred_config"):
+            class_._sa_raise_deferred_config()
+
+        raise orm_exc.UnmappedClassError(
+            class_,
+            msg="Class %s has a deferred mapping on it.  It is not yet "
+            "usable as a mapped class." % orm_exc._safe_cls_name(class_),
+        )
+
+    @classmethod
     def config_for_cls(cls, class_):
         return cls._configs[weakref.ref(class_)]