From ed3f2c617239668d74ad3d86aeda0ca2030a5933 Mon Sep 17 00:00:00 2001
From: Mike Bayer <mike_mp@zzzcomputing.com>
Date: Wed, 21 Apr 2021 10:39:09 -0400
Subject: [PATCH] Limit dc field logic to only fields that are definitely dc

Fixed regression where recent changes to support Python dataclasses had the
inadvertent effect that an ORM mapped class could not successfully override
the ``__new__()`` method.

In this case the "__new__" method comes out as staticmethod in
cls.__dict__ vs. a function in the metaclass dict_, so comparing
using identity fails.   I was hoping not to have too much
"dataclass" hardcoded, the logic here if it were generalized
to other attribute declaration systems there
would still have a flag that indicates an attribute is part of
the "special declaration system".

Fixes: #6331
Change-Id: Ia28a44fb57c668fa2fc5cd1ff38fd511f2c747e6
---
 doc/build/changelog/unreleased_14/6331.rst |  7 +++++++
 lib/sqlalchemy/orm/decl_base.py            | 16 +++++++++-------
 test/orm/declarative/test_basic.py         | 18 ++++++++++++++++++
 3 files changed, 34 insertions(+), 7 deletions(-)
 create mode 100644 doc/build/changelog/unreleased_14/6331.rst

diff --git a/doc/build/changelog/unreleased_14/6331.rst b/doc/build/changelog/unreleased_14/6331.rst
new file mode 100644
index 0000000000..0bcecaec90
--- /dev/null
+++ b/doc/build/changelog/unreleased_14/6331.rst
@@ -0,0 +1,7 @@
+.. change::
+    :tags: bug, orm, regression, declarative
+    :tickets: 6331
+
+    Fixed regression where recent changes to support Python dataclasses had the
+    inadvertent effect that an ORM mapped class could not successfully override
+    the ``__new__()`` method.
diff --git a/lib/sqlalchemy/orm/decl_base.py b/lib/sqlalchemy/orm/decl_base.py
index 5a5d98a958..b3444f26f4 100644
--- a/lib/sqlalchemy/orm/decl_base.py
+++ b/lib/sqlalchemy/orm/decl_base.py
@@ -410,7 +410,7 @@ class _ClassScanMapperConfig(_MapperConfig):
 
             def local_attributes_for_class():
                 for name, obj in vars(cls).items():
-                    yield name, obj
+                    yield name, obj, False
 
         else:
             field_names = set()
@@ -421,10 +421,10 @@ class _ClassScanMapperConfig(_MapperConfig):
                         field_names.add(field.name)
                         yield field.name, _as_dc_declaredattr(
                             field.metadata, sa_dataclass_metadata_key
-                        )
+                        ), True
                 for name, obj in vars(cls).items():
                     if name not in field_names:
-                        yield name, obj
+                        yield name, obj, False
 
         return local_attributes_for_class
 
@@ -455,7 +455,7 @@ class _ClassScanMapperConfig(_MapperConfig):
                     local_attributes_for_class, attribute_is_overridden
                 )
 
-            for name, obj in local_attributes_for_class():
+            for name, obj, is_dataclass in local_attributes_for_class():
                 if name == "__mapper_args__":
                     check_decl = _check_declared_props_nocascade(
                         obj, name, cls
@@ -567,15 +567,17 @@ class _ClassScanMapperConfig(_MapperConfig):
                     # however, check for some more common mistakes
                     else:
                         self._warn_for_decl_attributes(base, name, obj)
-                elif name not in dict_ or dict_[name] is not obj:
+                elif is_dataclass and (
+                    name not in dict_ or dict_[name] is not obj
+                ):
                     # here, we are definitely looking at the target class
                     # and not a superclass.   this is currently a
                     # dataclass-only path.  if the name is only
                     # a dataclass field and isn't in local cls.__dict__,
                     # put the object there.
-
                     # assert that the dataclass-enabled resolver agrees
                     # with what we are seeing
+
                     assert not attribute_is_overridden(name, obj)
 
                     if _is_declarative_props(obj):
@@ -607,7 +609,7 @@ class _ClassScanMapperConfig(_MapperConfig):
         column_copies = self.column_copies
         # copy mixin columns to the mapped class
 
-        for name, obj in attributes_for_class():
+        for name, obj, is_dataclass in attributes_for_class():
             if isinstance(obj, Column):
                 if attribute_is_overridden(name, obj):
                     # if column has been overridden
diff --git a/test/orm/declarative/test_basic.py b/test/orm/declarative/test_basic.py
index e7c8f188d9..ecc824391d 100644
--- a/test/orm/declarative/test_basic.py
+++ b/test/orm/declarative/test_basic.py
@@ -2282,6 +2282,24 @@ class DeclarativeTest(DeclarativeTestBase):
 
         assert not hasattr(Foo, "data_hybrid")
 
+    def test_classes_can_override_new(self):
+        class MyTable(Base):
+            __tablename__ = "my_table"
+            id = Column(Integer, primary_key=True)
+
+            def __new__(cls, *args, **kwargs):
+                return object.__new__(cls)
+
+            def some_method(self):
+                pass
+
+            @staticmethod
+            def some_static_method(self):
+                pass
+
+        mt = MyTable(id=5)
+        eq_(mt.id, 5)
+
     @testing.requires.python36
     def test_kw_support_in_declarative_meta_init(self):
         # This will not fail if DeclarativeMeta __init__ supports **kw
-- 
2.47.3