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.

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)