Add workaround for model sometimes returning a numeric .id on create

See issue #6.

This works around the following upstream Django issue, which affects
versions pre-4.0: https://code.djangoproject.com/ticket/32442

The fix is a bit ugly (installs a signal handler; which runs in non-deterministic
order alongside any others that have been installed), but I don't see a better one.

README.md

@@ -83,6 +83,8 @@ This package supports and is tested against the latest patch versions of:
 
 All database backends are tested with the latest versions of their drivers. SQLite is also tested on GitHub Actions' latest macOS virtual environment.
 
+**Note:** Django 4.x is recommended due to an [upstream bug](https://code.djangoproject.com/ticket/32442) that is present in 3.x. See [#6](https://github.com/mik3y/django-spicy-id/issues/6) for further details.
+
 ### Instructions
 
 ```

django_spicy_id/fields.py

@@ -2,8 +2,10 @@
 import re
 import secrets
 
+import django
 from django.core.exceptions import ImproperlyConfigured
 from django.db import models
+from django.db.models.signals import post_save
 from django.db.utils import ProgrammingError
 
 from django_spicy_id.errors import MalformedSpicyIdError
@@ -182,6 +184,37 @@ def deconstruct(self):
             del kwargs["default"]
         return name, path, args, kwargs
 
+    def contribute_to_class(self, cls, name, **kwargs):
+        super().contribute_to_class(cls, name, **kwargs)
+
+        # Special case: Register a signal handler when this row is created.
+        # Workaround for issue #6 / Django issue 32442.
+        #
+        # When the DB connection does not support `can_return_columns_from_insert`,
+        # the raw row value is returned by Django on insert. This causes the caller
+        # to receive a numeric value when the row is inserted, due to the upstream
+        # bug (value does flow through `Field.from_db_value()`).
+        #
+        # https://code.djangoproject.com/ticket/32442
+
+        # Not needed in Django >= 4.
+        if django.VERSION >= (4, 0):
+            return
+
+        # Only affects primary keys.
+        if not self.primary_key:
+            return
+
+        def spicy_id_create_handler(sender, instance, created, raw, **kwargs):
+            if not created or raw:
+                return
+            nonlocal name
+            val = getattr(instance, name)
+            if isinstance(val, int):
+                setattr(instance, name, self.to_python(val))
+
+        post_save.connect(spicy_id_create_handler, sender=cls, weak=False)
+
 
 class SpicyBigAutoField(BaseSpicyAutoField, models.BigAutoField):
     """A Spicy ID field that is backed by a standard 64-bit Django BigAutoField."""

django_spicy_id/tests/fields_test.py

@@ -209,12 +209,12 @@ def test_base62_model_create_by_integer(self):
     @mock.patch("secrets.randbelow")
     def test_randomize_sets_pk_to_a_string(self, mock_secrets_randbelow):
         """Ensures that when `randomize` is used, the value set is a string not a number."""
-        model = models.Base62Model_WithRandomize
+        model = models.SpicyAutoFieldModel_WithRandomize
 
         mock_secrets_randbelow.return_value = 1
         o = model()
-        self.assertEqual("ex_2", o.pk)
+        self.assertEqual("ex_2", o.id)
         self.assertTrue(o._state.adding)
         o.save()
-        self.assertEqual("ex_2", o.pk)
+        self.assertEqual("ex_2", o.id)
         self.assertFalse(o._state.adding)

django_spicy_id/tests/models.py

@@ -1,6 +1,6 @@
 from django.db import models
 
-from django_spicy_id import SpicyBigAutoField
+from django_spicy_id import SpicyAutoField, SpicyBigAutoField
 
 
 class Model_WithDefaults(models.Model):
@@ -31,3 +31,7 @@ class Base62Model_WithRandomize(models.Model):
 
 class HexModel_WithRandomize(models.Model):
     id = SpicyBigAutoField("ex", primary_key=True, encoding="hex", randomize=True)
+
+
+class SpicyAutoFieldModel_WithRandomize(models.Model):
+    id = SpicyAutoField("ex", primary_key=True, encoding="hex", randomize=True)