Fixed #29979, Refs #17337 -- Extracted AutoField field logic into a mixin and refactored AutoFields.

This reduces duplication by allowing AutoField, BigAutoField and
SmallAutoField to inherit from IntegerField, BigIntegerField and
SmallIntegerField respectively. Doing so also allows for enabling the
max_length warning check and minimum/maximum value validation for auto
fields, as well as providing a mixin that can be used for other possible
future auto field types such as a theoretical UUIDAutoField.

8 files changed, 171 insertions, 128 deletions

diff --git a/django/db/backends/base/operations.py b/django/db/backends/base/operations.py
index d17dc8dcce..76abc6dcb2 100644
--- a/django/db/backends/base/operations.py
+++ b/django/db/backends/base/operations.py
@@ -26,6 +26,9 @@ class BaseDatabaseOperations:
         'BigIntegerField': (-9223372036854775808, 9223372036854775807),
         'PositiveSmallIntegerField': (0, 32767),
         'PositiveIntegerField': (0, 2147483647),
+        'SmallAutoField': (-32768, 32767),
+        'AutoField': (-2147483648, 2147483647),
+        'BigAutoField': (-9223372036854775808, 9223372036854775807),
     }
     set_operators = {
         'union': 'UNION',
diff --git a/django/db/backends/oracle/operations.py b/django/db/backends/oracle/operations.py
index 016d4c9a59..3cad02fa41 100644
--- a/django/db/backends/oracle/operations.py
+++ b/django/db/backends/oracle/operations.py
@@ -16,13 +16,18 @@ from .utils import BulkInsertMapper, InsertVar, Oracle_datetime
 
 
 class DatabaseOperations(BaseDatabaseOperations):
-    # Oracle uses NUMBER(11) and NUMBER(19) for integer fields.
+    # Oracle uses NUMBER(5), NUMBER(11), and NUMBER(19) for integer fields.
+    # SmallIntegerField uses NUMBER(11) instead of NUMBER(5), which is used by
+    # SmallAutoField, to preserve backward compatibility.
     integer_field_ranges = {
         'SmallIntegerField': (-99999999999, 99999999999),
         'IntegerField': (-99999999999, 99999999999),
         'BigIntegerField': (-9999999999999999999, 9999999999999999999),
         'PositiveSmallIntegerField': (0, 99999999999),
         'PositiveIntegerField': (0, 99999999999),
+        'SmallAutoField': (-99999, 99999),
+        'AutoField': (-99999999999, 99999999999),
+        'BigAutoField': (-9999999999999999999, 9999999999999999999),
     }
     set_operators = {**BaseDatabaseOperations.set_operators, 'difference': 'MINUS'}
 
diff --git a/django/db/models/fields/__init__.py b/django/db/models/fields/__init__.py
index 1aad845470..6e924a4adf 100644
--- a/django/db/models/fields/__init__.py
+++ b/django/db/models/fields/__init__.py
@@ -898,110 +898,6 @@ class Field(RegisterLookupMixin):
         return getattr(obj, self.attname)
 
 
-class AutoField(Field):
-    description = _("Integer")
-
-    empty_strings_allowed = False
-    default_error_messages = {
-        'invalid': _('“%(value)s” value must be an integer.'),
-    }
-
-    def __init__(self, *args, **kwargs):
-        kwargs['blank'] = True
-        super().__init__(*args, **kwargs)
-
-    def check(self, **kwargs):
-        return [
-            *super().check(**kwargs),
-            *self._check_primary_key(),
-        ]
-
-    def _check_primary_key(self):
-        if not self.primary_key:
-            return [
-                checks.Error(
-                    'AutoFields must set primary_key=True.',
-                    obj=self,
-                    id='fields.E100',
-                ),
-            ]
-        else:
-            return []
-
-    def deconstruct(self):
-        name, path, args, kwargs = super().deconstruct()
-        del kwargs['blank']
-        kwargs['primary_key'] = True
-        return name, path, args, kwargs
-
-    def get_internal_type(self):
-        return "AutoField"
-
-    def to_python(self, value):
-        if value is None:
-            return value
-        try:
-            return int(value)
-        except (TypeError, ValueError):
-            raise exceptions.ValidationError(
-                self.error_messages['invalid'],
-                code='invalid',
-                params={'value': value},
-            )
-
-    def rel_db_type(self, connection):
-        return IntegerField().db_type(connection=connection)
-
-    def validate(self, value, model_instance):
-        pass
-
-    def get_db_prep_value(self, value, connection, prepared=False):
-        if not prepared:
-            value = self.get_prep_value(value)
-            value = connection.ops.validate_autopk_value(value)
-        return value
-
-    def get_prep_value(self, value):
-        from django.db.models.expressions import OuterRef
-        value = super().get_prep_value(value)
-        if value is None or isinstance(value, OuterRef):
-            return value
-        try:
-            return int(value)
-        except (TypeError, ValueError) as e:
-            raise e.__class__(
-                "Field '%s' expected a number but got %r." % (self.name, value),
-            ) from e
-
-    def contribute_to_class(self, cls, name, **kwargs):
-        assert not cls._meta.auto_field, "Model %s can't have more than one AutoField." % cls._meta.label
-        super().contribute_to_class(cls, name, **kwargs)
-        cls._meta.auto_field = self
-
-    def formfield(self, **kwargs):
-        return None
-
-
-class BigAutoField(AutoField):
-    description = _("Big (8 byte) integer")
-
-    def get_internal_type(self):
-        return "BigAutoField"
-
-    def rel_db_type(self, connection):
-        return BigIntegerField().db_type(connection=connection)
-
-
-class SmallAutoField(AutoField):
-    description = _('Small integer')
-
-    def get_internal_type(self):
-        return 'SmallAutoField'
-
-    def rel_db_type(self, connection):
-        return SmallIntegerField().db_type(connection=connection)
-
-
 class BooleanField(Field):
     empty_strings_allowed = False
     default_error_messages = {
@@ -2395,3 +2291,113 @@ class UUIDField(Field):
             'form_class': forms.UUIDField,
             **kwargs,
         })
+
+
+class AutoFieldMixin:
+
+    def __init__(self, *args, **kwargs):
+        kwargs['blank'] = True
+        super().__init__(*args, **kwargs)
+
+    def check(self, **kwargs):
+        return [
+            *super().check(**kwargs),
+            *self._check_primary_key(),
+        ]
+
+    def _check_primary_key(self):
+        if not self.primary_key:
+            return [
+                checks.Error(
+                    'AutoFields must set primary_key=True.',
+                    obj=self,
+                    id='fields.E100',
+                ),
+            ]
+        else:
+            return []
+
+    def deconstruct(self):
+        name, path, args, kwargs = super().deconstruct()
+        del kwargs['blank']
+        kwargs['primary_key'] = True
+        return name, path, args, kwargs
+
+    def validate(self, value, model_instance):
+        pass
+
+    def get_db_prep_value(self, value, connection, prepared=False):
+        if not prepared:
+            value = self.get_prep_value(value)
+            value = connection.ops.validate_autopk_value(value)
+        return value
+
+    def get_prep_value(self, value):
+        from django.db.models.expressions import OuterRef
+        return value if isinstance(value, OuterRef) else super().get_prep_value(value)
+
+    def contribute_to_class(self, cls, name, **kwargs):
+        assert not cls._meta.auto_field, (
+            "Model %s can't have more than one auto-generated field."
+            % cls._meta.label
+        )
+        super().contribute_to_class(cls, name, **kwargs)
+        cls._meta.auto_field = self
+
+    def formfield(self, **kwargs):
+        return None
+
+
+class AutoFieldMeta(type):
+    """
+    Metaclass to maintain backward inheritance compatibility for AutoField.
+
+    It is intended that AutoFieldMixin become public API when it is possible to
+    create a non-integer automatically-generated field using column defaults
+    stored in the database.
+
+    In many areas Django also relies on using isinstance() to check for an
+    automatically-generated field as a subclass of AutoField. A new flag needs
+    to be implemented on Field to be used instead.
+
+    When these issues have been addressed, this metaclass could be used to
+    deprecate inheritance from AutoField and use of isinstance() with AutoField
+    for detecting automatically-generated fields.
+    """
+
+    @property
+    def _subclasses(self):
+        return (BigAutoField, SmallAutoField)
+
+    def __instancecheck__(self, instance):
+        return isinstance(instance, self._subclasses) or super().__instancecheck__(instance)
+
+    def __subclasscheck__(self, subclass):
+        return subclass in self._subclasses or super().__subclasscheck__(subclass)
+
+
+class AutoField(AutoFieldMixin, IntegerField, metaclass=AutoFieldMeta):
+
+    def get_internal_type(self):
+        return 'AutoField'
+
+    def rel_db_type(self, connection):
+        return IntegerField().db_type(connection=connection)
+
+
+class BigAutoField(AutoFieldMixin, BigIntegerField):
+
+    def get_internal_type(self):
+        return 'BigAutoField'
+
+    def rel_db_type(self, connection):
+        return BigIntegerField().db_type(connection=connection)
+
+
+class SmallAutoField(AutoFieldMixin, SmallIntegerField):
+
+    def get_internal_type(self):
+        return 'SmallAutoField'
+
+    def rel_db_type(self, connection):
+        return SmallIntegerField().db_type(connection=connection)
diff --git a/docs/releases/3.0.txt b/docs/releases/3.0.txt
index 2e345f6fe5..5bb0691363 100644
--- a/docs/releases/3.0.txt
+++ b/docs/releases/3.0.txt
@@ -304,6 +304,12 @@ Models
   a certain (database-dependent) limit. Values from ``1`` to ``32767`` are safe
   in all databases supported by Django.
 
+* :class:`~django.db.models.AutoField`,
+  :class:`~django.db.models.BigAutoField`, and
+  :class:`~django.db.models.SmallAutoField` now inherit from
+  ``IntegerField``, ``BigIntegerField`` and ``SmallIntegerField`` respectively.
+  System checks and validators are now also properly inherited.
+
 * :attr:`.FileField.upload_to` now supports :class:`pathlib.Path`.
 
 Requests and Responses
@@ -402,6 +408,11 @@ backends.
 * ``DatabaseOperations.return_insert_id()`` now requires an additional
   ``field`` argument with the model field.
 
+* Entries for ``AutoField``, ``BigAutoField``, and ``SmallAutoField`` are added
+  to  ``DatabaseOperations.integer_field_ranges`` to support the integer range
+  validators on these field types. Third-party backends may need to customize
+  the default entries.
+
 :mod:`django.contrib.admin`
 ---------------------------
 
diff --git a/tests/invalid_models_tests/test_ordinary_fields.py b/tests/invalid_models_tests/test_ordinary_fields.py
index a3231ee36a..5bb1847a70 100644
--- a/tests/invalid_models_tests/test_ordinary_fields.py
+++ b/tests/invalid_models_tests/test_ordinary_fields.py
@@ -38,6 +38,21 @@ class AutoFieldTests(SimpleTestCase):
             ),
         ])
 
+    def test_max_length_warning(self):
+        class Model(models.Model):
+            auto = models.AutoField(primary_key=True, max_length=2)
+
+        field = Model._meta.get_field('auto')
+        self.assertEqual(field.check(), [
+            DjangoWarning(
+                "'max_length' is ignored when used with %s."
+                % field.__class__.__name__,
+                hint="Remove 'max_length' from field",
+                obj=field,
+                id='fields.W122',
+            ),
+        ])
+
 
 @isolate_apps('invalid_models_tests')
 class BinaryFieldTests(SimpleTestCase):
diff --git a/tests/model_fields/test_autofield.py b/tests/model_fields/test_autofield.py
index 73220ae62b..db8e79e34b 100644
--- a/tests/model_fields/test_autofield.py
+++ b/tests/model_fields/test_autofield.py
@@ -1,32 +1,32 @@
-from django.test import TestCase
+from django.db import models
+from django.test import SimpleTestCase
 
 from .models import AutoModel, BigAutoModel, SmallAutoModel
+from .test_integerfield import (
+    BigIntegerFieldTests, IntegerFieldTests, SmallIntegerFieldTests,
+)
 
 
-class AutoFieldTests(TestCase):
+class AutoFieldTests(IntegerFieldTests):
     model = AutoModel
 
-    def test_invalid_value(self):
-        tests = [
-            (TypeError, ()),
-            (TypeError, []),
-            (TypeError, {}),
-            (TypeError, set()),
-            (TypeError, object()),
-            (TypeError, complex()),
-            (ValueError, 'non-numeric string'),
-            (ValueError, b'non-numeric byte-string'),
-        ]
-        for exception, value in tests:
-            with self.subTest(value=value):
-                msg = "Field 'value' expected a number but got %r." % (value,)
-                with self.assertRaisesMessage(exception, msg):
-                    self.model.objects.create(value=value)
 
-
-class BigAutoFieldTests(AutoFieldTests):
+class BigAutoFieldTests(BigIntegerFieldTests):
     model = BigAutoModel
 
 
-class SmallAutoFieldTests(AutoFieldTests):
+class SmallAutoFieldTests(SmallIntegerFieldTests):
     model = SmallAutoModel
+
+
+class AutoFieldInheritanceTests(SimpleTestCase):
+
+    def test_isinstance_of_autofield(self):
+        for field in (models.BigAutoField, models.SmallAutoField):
+            with self.subTest(field.__name__):
+                self.assertIsInstance(field(), models.AutoField)
+
+    def test_issubclass_of_autofield(self):
+        for field in (models.BigAutoField, models.SmallAutoField):
+            with self.subTest(field.__name__):
+                self.assertTrue(issubclass(field, models.AutoField))
diff --git a/tests/model_fields/test_integerfield.py b/tests/model_fields/test_integerfield.py
index c72cb48015..c0bb7627cf 100644
--- a/tests/model_fields/test_integerfield.py
+++ b/tests/model_fields/test_integerfield.py
@@ -125,7 +125,7 @@ class IntegerFieldTests(TestCase):
                         ranged_value_field.run_validators(max_backend_value + 1)
 
     def test_types(self):
-        instance = self.model(value=0)
+        instance = self.model(value=1)
         self.assertIsInstance(instance.value, int)
         instance.save()
         self.assertIsInstance(instance.value, int)
diff --git a/tests/validation/models.py b/tests/validation/models.py
index 953370bc37..e8e18cfbce 100644
--- a/tests/validation/models.py
+++ b/tests/validation/models.py
@@ -130,4 +130,7 @@ try:
         auto2 = models.AutoField(primary_key=True)
 except AssertionError as exc:
     assertion_error = exc
-assert str(assertion_error) == "Model validation.MultipleAutoFields can't have more than one AutoField."
+assert str(assertion_error) == (
+    "Model validation.MultipleAutoFields can't have more than one "
+    "auto-generated field."
+)