implement dataclass_transforms

Implement a new means of creating a mapped dataclass where
instead of applying the `@dataclass` decorator distinctly,
the declarative process itself can create the dataclass.

MapperProperty and MappedColumn objects themselves take
the place of the dataclasses.Field object when constructing
the class.

The overall approach is made possible at the typing level
using pep-681 dataclass transforms [1].

This new approach should be able to completely supersede the
previous "dataclasses" approach of embedding metadata into
Field() objects, which remains a mutually exclusive declarative
setup style (mixing them introduces new issues that are not worth
solving).

[1] https://peps.python.org/pep-0681/#transform-descriptor-types-example

Fixes: #7642
Change-Id: I6ba88a87c5df38270317b4faf085904d91c8a63c

1 files changed, 24 insertions, 1 deletions

diff --git a/lib/sqlalchemy/testing/fixtures.py b/lib/sqlalchemy/testing/fixtures.py
index 53f76f3ce..d4e4d2dca 100644
--- a/lib/sqlalchemy/testing/fixtures.py
+++ b/lib/sqlalchemy/testing/fixtures.py
@@ -25,6 +25,7 @@ from .. import event
 from .. import util
 from ..orm import declarative_base
 from ..orm import DeclarativeBase
+from ..orm import MappedAsDataclass
 from ..orm import registry
 from ..schema import sort_tables_and_constraints
 
@@ -90,7 +91,14 @@ class TestBase:
 
     @config.fixture()
     def registry(self, metadata):
-        reg = registry(metadata=metadata)
+        reg = registry(
+            metadata=metadata,
+            type_annotation_map={
+                str: sa.String().with_variant(
+                    sa.String(50), "mysql", "mariadb"
+                )
+            },
+        )
         yield reg
         reg.dispose()
 
@@ -109,6 +117,21 @@ class TestBase:
         yield Base
         Base.registry.dispose()
 
+    @config.fixture
+    def dc_decl_base(self, metadata):
+        _md = metadata
+
+        class Base(MappedAsDataclass, DeclarativeBase):
+            metadata = _md
+            type_annotation_map = {
+                str: sa.String().with_variant(
+                    sa.String(50), "mysql", "mariadb"
+                )
+            }
+
+        yield Base
+        Base.registry.dispose()
+
     @config.fixture()
     def future_connection(self, future_engine, connection):
         # integrate the future_engine and connection fixtures so