Allow Declarative to extract class attr from field

Added an alternate resolution scheme to Declarative that will extract the
SQLAlchemy column or mapped property from the "metadata" dictionary of a
dataclasses.Field object.  This allows full declarative mappings to be
combined with dataclass fields.

Fixes: #5745
Change-Id: I1165bc025246a4cb9fc099b1b7c46a6b0f799b23

2 files changed, 17 insertions, 0 deletions

diff --git a/lib/sqlalchemy/util/__init__.py b/lib/sqlalchemy/util/__init__.py
index f4363d03c..2e3f68722 100644
--- a/lib/sqlalchemy/util/__init__.py
+++ b/lib/sqlalchemy/util/__init__.py
@@ -57,6 +57,7 @@ from .compat import byte_buffer  # noqa
 from .compat import callable  # noqa
 from .compat import cmp  # noqa
 from .compat import cpython  # noqa
+from .compat import dataclass_fields  # noqa
 from .compat import decode_backslashreplace  # noqa
 from .compat import dottedgetter  # noqa
 from .compat import has_refcount_gc  # noqa
diff --git a/lib/sqlalchemy/util/compat.py b/lib/sqlalchemy/util/compat.py
index e8c488047..77c913640 100644
--- a/lib/sqlalchemy/util/compat.py
+++ b/lib/sqlalchemy/util/compat.py
@@ -421,6 +421,22 @@ else:
     import collections as collections_abc  # noqa
 
 
+if py37:
+    import dataclasses
+
+    def dataclass_fields(cls):
+        if dataclasses.is_dataclass(cls):
+            return dataclasses.fields(cls)
+        else:
+            return []
+
+
+else:
+
+    def dataclass_fields(cls):
+        return []
+
+
 def raise_from_cause(exception, exc_info=None):
     r"""legacy.  use raise\_()"""