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, 11 insertions, 0 deletions

diff --git a/lib/sqlalchemy/ext/automap.py b/lib/sqlalchemy/ext/automap.py
index 55ab325a3..5fa4e8822 100644
--- a/lib/sqlalchemy/ext/automap.py
+++ b/lib/sqlalchemy/ext/automap.py
@@ -517,6 +517,7 @@ from .declarative import declarative_base as _declarative_base
 from .declarative.base import _DeferredMapperConfig
 from .. import util
 from ..orm import backref
+from ..orm import exc as orm_exc
 from ..orm import interfaces
 from ..orm import relationship
 from ..orm.mapper import _CONFIGURE_MUTEX
@@ -841,6 +842,16 @@ class AutomapBase(object):
 
     """
 
+    @classmethod
+    def _sa_raise_deferred_config(cls):
+        raise orm_exc.UnmappedClassError(
+            cls,
+            msg="Class %s is a subclass of AutomapBase.  "
+            "Mappings are not produced until the .prepare() "
+            "method is called on the class hierarchy."
+            % orm_exc._safe_cls_name(cls),
+        )
+
 
 def automap_base(declarative_base=None, **kw):
     r"""Produce a declarative automap base.