From 8f720e1529af2b4810b0ae5379e13b343155eded Mon Sep 17 00:00:00 2001
From: Federico Caselli <cfederico87@gmail.com>
Date: Sat, 3 Jun 2023 12:40:00 +0200
Subject: [PATCH] Support dataclass default with init=False

Fixed an issue where generating dataclasses fields that specified a
default`` value and set ``init=False`` would not work.
The dataclasses behavior in this case is to set the default
value on the class, that's not compatible with the descriptors used
by SQLAlchemy. To support this case the default is transformed to
a ``default_factory`` when generating the dataclass.

Fixes: #9879
Change-Id: I5151d388232eacd506a100ba18ce26970bf83cf3
---
 doc/build/changelog/unreleased_20/9879.rst | 10 +++++++
 lib/sqlalchemy/orm/interfaces.py           | 10 +++++++
 test/orm/declarative/test_dc_transforms.py | 32 ++++++++++++++++++++++
 3 files changed, 52 insertions(+)
 create mode 100644 doc/build/changelog/unreleased_20/9879.rst

diff --git a/doc/build/changelog/unreleased_20/9879.rst b/doc/build/changelog/unreleased_20/9879.rst
new file mode 100644
index 0000000000..d1112305ba
--- /dev/null
+++ b/doc/build/changelog/unreleased_20/9879.rst
@@ -0,0 +1,10 @@
+.. change::
+    :tags: bug, orm, dataclasses
+    :tickets: 9879
+
+    Fixed an issue where generating dataclasses fields that specified a
+    ``default`` value and set ``init=False`` would not work.
+    The dataclasses behavior in this case is to set the default
+    value on the class, that's not compatible with the descriptors used
+    by SQLAlchemy. To support this case the default is transformed to
+    a ``default_factory`` when generating the dataclass.
diff --git a/lib/sqlalchemy/orm/interfaces.py b/lib/sqlalchemy/orm/interfaces.py
index 4da8f63e68..d9df2b3b19 100644
--- a/lib/sqlalchemy/orm/interfaces.py
+++ b/lib/sqlalchemy/orm/interfaces.py
@@ -217,6 +217,16 @@ class _AttributeOptions(NamedTuple):
         if self.dataclasses_kw_only is not _NoArg.NO_ARG:
             kw["kw_only"] = self.dataclasses_kw_only
 
+        if (
+            "init" in kw
+            and not kw["init"]
+            and "default" in kw
+            and "default_factory" not in kw  # illegal but let field raise
+        ):
+            # fix for #9879
+            default = kw.pop("default")
+            kw["default_factory"] = lambda: default
+
         return dataclasses.field(**kw)
 
     @classmethod
diff --git a/test/orm/declarative/test_dc_transforms.py b/test/orm/declarative/test_dc_transforms.py
index 576ee7fbfe..678dc51a27 100644
--- a/test/orm/declarative/test_dc_transforms.py
+++ b/test/orm/declarative/test_dc_transforms.py
@@ -758,6 +758,38 @@ class DCTransformsTest(AssertsCompiledSQL, fixtures.TestBase):
 
         is_true(isinstance(ec.error.__cause__, TypeError))
 
+    def test_dataclass_default(self, dc_decl_base):
+        """test for #9879"""
+
+        def c10():
+            return 10
+
+        def c20():
+            return 20
+
+        class A(dc_decl_base):
+            __tablename__ = "a"
+            id: Mapped[int] = mapped_column(primary_key=True)
+            def_init: Mapped[int] = mapped_column(default=42)
+            call_init: Mapped[int] = mapped_column(default_factory=c10)
+            def_no_init: Mapped[int] = mapped_column(default=13, init=False)
+            call_no_init: Mapped[int] = mapped_column(
+                default_factory=c20, init=False
+            )
+
+        a = A(id=100)
+        eq_(a.def_init, 42)
+        eq_(a.call_init, 10)
+        eq_(a.def_no_init, 13)
+        eq_(a.call_no_init, 20)
+
+        fields = {f.name: f for f in dataclasses.fields(A)}
+        eq_(fields["def_init"].default, 42)
+        eq_(fields["call_init"].default_factory, c10)
+        eq_(fields["def_no_init"].default, dataclasses.MISSING)
+        ne_(fields["def_no_init"].default_factory, dataclasses.MISSING)
+        eq_(fields["call_no_init"].default_factory, c20)
+
 
 class RelationshipDefaultFactoryTest(fixtures.TestBase):
     def test_list(self, dc_decl_base: Type[MappedAsDataclass]):
-- 
2.47.2