Bypass declared_attr w/ a custom wrapper for lambda criteria

Fixed bug in new :func:`_orm.with_loader_criteria` feature where using a
mixin class with :func:`_orm.declared_attr` on an attribute that were
accessed inside the custom lambda would emit a warning regarding using an
unmapped declared attr, when the lambda callable were first initialized.
This warning is now prevented using special instrumentation for this
lambda initialization step.

Fixes: #6320
Change-Id: I18ce0c662131f2e683b84caa38c01b2182eb210b

1 files changed, 31 insertions, 3 deletions

diff --git a/lib/sqlalchemy/orm/util.py b/lib/sqlalchemy/orm/util.py
index a30f085aa..65fdcf9fc 100644
--- a/lib/sqlalchemy/orm/util.py
+++ b/lib/sqlalchemy/orm/util.py
@@ -874,6 +874,32 @@ class AliasedInsp(
             return "aliased(%s)" % (self._target.__name__,)
 
 
+class _WrapUserEntity(object):
+    """A wrapper used within the loader_criteria lambda caller so that
+    we can bypass declared_attr descriptors on unmapped mixins, which
+    normally emit a warning for such use.
+
+    might also be useful for other per-lambda instrumentations should
+    the need arise.
+
+    """
+
+    def __init__(self, subject):
+        self.subject = subject
+
+    @util.preload_module("sqlalchemy.orm.decl_api")
+    def __getattribute__(self, name):
+        decl_api = util.preloaded.orm.decl_api
+
+        subject = object.__getattribute__(self, "subject")
+        if name in subject.__dict__ and isinstance(
+            subject.__dict__[name], decl_api.declared_attr
+        ):
+            return subject.__dict__[name].fget(subject)
+        else:
+            return getattr(subject, name)
+
+
 class LoaderCriteriaOption(CriteriaOption):
     """Add additional WHERE criteria to the load for all occurrences of
     a particular entity.
@@ -1033,9 +1059,11 @@ class LoaderCriteriaOption(CriteriaOption):
                 where_criteria,
                 roles.WhereHavingRole,
                 lambda_args=(
-                    self.root_entity
-                    if self.root_entity is not None
-                    else self.entity.entity,
+                    _WrapUserEntity(
+                        self.root_entity
+                        if self.root_entity is not None
+                        else self.entity.entity,
+                    ),
                 ),
                 opts=lambdas.LambdaOptions(
                     track_closure_variables=track_closure_variables