allow legacy forms with __allow_unmapped__

Improved support for legacy 1.4 mappings that use annotations which don't
include ``Mapped[]``, by ensuring the ``__allow_unmapped__`` attribute can
be used to allow such legacy annotations to pass through Annotated
Declarative without raising an error and without being interpreted in an
ORM runtime context. Additionally improved the error message generated when
this condition is detected, and added more documentation for how this
situation should be handled. Unfortunately the 1.4 WARN_SQLALCHEMY_20
migration warning cannot detect this particular configurational issue at
runtime with its current architecture.

Fixes: #8692
Change-Id: I5c642bcc1ebb7816f9470ec9bb0951550b6d55f1

1 files changed, 27 insertions, 9 deletions

diff --git a/lib/sqlalchemy/orm/decl_base.py b/lib/sqlalchemy/orm/decl_base.py
index ef2c2f3c9..4e79ecc6f 100644
--- a/lib/sqlalchemy/orm/decl_base.py
+++ b/lib/sqlalchemy/orm/decl_base.py
@@ -1199,6 +1199,11 @@ class _ClassScanMapperConfig(_MapperConfig):
             cls, "_sa_decl_prepare_nocascade", strict=True
         )
 
+        expect_annotations_wo_mapped = (
+            self.allow_unmapped_annotations
+            or self.is_dataclass_prior_to_mapping
+        )
+
         for k in list(collected_attributes):
 
             if k in ("__table__", "__tablename__", "__mapper_args__"):
@@ -1275,15 +1280,28 @@ class _ClassScanMapperConfig(_MapperConfig):
                     ) = self.collected_annotations.get(
                         k, (None, None, None, False, None)
                     )
-                    value.declarative_scan(
-                        self.registry,
-                        cls,
-                        k,
-                        mapped_container,
-                        annotation,
-                        extracted_mapped_annotation,
-                        is_dataclass,
-                    )
+
+                    # issue #8692 - don't do any annotation interpretation if
+                    # an annotation were present and a container such as
+                    # Mapped[] etc. were not used.  If annotation is None,
+                    # do declarative_scan so that the property can raise
+                    # for required
+                    if mapped_container is not None or annotation is None:
+                        value.declarative_scan(
+                            self.registry,
+                            cls,
+                            k,
+                            mapped_container,
+                            annotation,
+                            extracted_mapped_annotation,
+                            is_dataclass,
+                        )
+                    else:
+                        # assert that we were expecting annotations
+                        # without Mapped[] were going to be passed.
+                        # otherwise an error should have been raised
+                        # by util._extract_mapped_subtype before we got here.
+                        assert expect_annotations_wo_mapped
 
                 if (
                     isinstance(value, (MapperProperty, _MapsColumns))