Fixed #30657 -- Allowed customizing Field's descriptors with a descriptor_class attribute.

Allows model fields to override the descriptor class used on the model
instance attribute.

2 files changed, 45 insertions, 2 deletions

diff --git a/tests/field_subclassing/fields.py b/tests/field_subclassing/fields.py
index 4eb170116f..1e255ba9cd 100644
--- a/tests/field_subclassing/fields.py
+++ b/tests/field_subclassing/fields.py
@@ -1,6 +1,26 @@
 from django.db import models
+from django.db.models.query_utils import DeferredAttribute
 
 
 class CustomTypedField(models.TextField):
     def db_type(self, connection):
         return 'custom_field'
+
+
+class CustomDeferredAttribute(DeferredAttribute):
+    def __get__(self, instance, cls=None):
+        self._count_call(instance, 'get')
+        return super().__get__(instance, cls)
+
+    def __set__(self, instance, value):
+        self._count_call(instance, 'set')
+        instance.__dict__[self.field.attname] = value
+
+    def _count_call(self, instance, get_or_set):
+        count_attr = '_%s_%s_count' % (self.field.attname, get_or_set)
+        count = getattr(instance, count_attr, 0)
+        setattr(instance, count_attr, count + 1)
+
+
+class CustomDescriptorField(models.CharField):
+    descriptor_class = CustomDeferredAttribute
diff --git a/tests/field_subclassing/tests.py b/tests/field_subclassing/tests.py
index a1371cab42..6d62a5da0e 100644
--- a/tests/field_subclassing/tests.py
+++ b/tests/field_subclassing/tests.py
@@ -1,7 +1,7 @@
-from django.db import connection
+from django.db import connection, models
 from django.test import SimpleTestCase
 
-from .fields import CustomTypedField
+from .fields import CustomDescriptorField, CustomTypedField
 
 
 class TestDbType(SimpleTestCase):
@@ -9,3 +9,26 @@ class TestDbType(SimpleTestCase):
     def test_db_parameters_respects_db_type(self):
         f = CustomTypedField()
         self.assertEqual(f.db_parameters(connection)['type'], 'custom_field')
+
+
+class DescriptorClassTest(SimpleTestCase):
+    def test_descriptor_class(self):
+        class CustomDescriptorModel(models.Model):
+            name = CustomDescriptorField(max_length=32)
+
+        m = CustomDescriptorModel()
+        self.assertFalse(hasattr(m, '_name_get_count'))
+        # The field is set to its default in the model constructor.
+        self.assertEqual(m._name_set_count, 1)
+        m.name = 'foo'
+        self.assertFalse(hasattr(m, '_name_get_count'))
+        self.assertEqual(m._name_set_count, 2)
+        self.assertEqual(m.name, 'foo')
+        self.assertEqual(m._name_get_count, 1)
+        self.assertEqual(m._name_set_count, 2)
+        m.name = 'bar'
+        self.assertEqual(m._name_get_count, 1)
+        self.assertEqual(m._name_set_count, 3)
+        self.assertEqual(m.name, 'bar')
+        self.assertEqual(m._name_get_count, 2)
+        self.assertEqual(m._name_set_count, 3)