Fixed #17653 -- Allowed using zero as AutoFields value on MySQL if NO_AUTO_VALUE_ON_ZERO SQL mode is enabled.

django/db/backends/base/features.py

@@ -100,7 +100,7 @@ class BaseDatabaseFeatures:
     # The database's limit on the number of query parameters.
     max_query_params = None
 
-    # Can an object have an autoincrement primary key of 0? MySQL says No.
+    # Can an object have an autoincrement primary key of 0?
     allows_auto_pk_0 = True
 
     # Do we need to NULL a ForeignKey out, or can the constraint check be

django/db/backends/mysql/features.py

@@ -17,7 +17,6 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     supports_index_column_ordering = False
     supports_timezones = False
     requires_explicit_null_ordering_when_grouping = True
-    allows_auto_pk_0 = False
     can_release_savepoints = True
     atomic_transactions = False
     can_clone_databases = True
@@ -51,6 +50,14 @@ class DatabaseFeatures(BaseDatabaseFeatures):
         "Internal method used in Django tests. Don't rely on this from your code"
         return self.connection.mysql_server_data['default_storage_engine']
 
+    @cached_property
+    def allows_auto_pk_0(self):
+        """
+        Autoincrement primary key can be set to 0 if it doesn't generate new
+        autoincrement values.
+        """
+        return 'NO_AUTO_VALUE_ON_ZERO' in self.connection.sql_mode
+
     @cached_property
     def update_can_self_select(self):
         return self.connection.mysql_is_mariadb and self.connection.mysql_version >= (10, 3, 2)

django/db/backends/mysql/operations.py

@@ -227,8 +227,9 @@ class DatabaseOperations(BaseDatabaseOperations):
         ]
 
     def validate_autopk_value(self, value):
-        # MySQLism: zero in AUTO_INCREMENT field does not work. Refs #17653.
-        if value == 0:
+        # Zero in AUTO_INCREMENT field does not work without the
+        # NO_AUTO_VALUE_ON_ZERO SQL mode.
+        if value == 0 and not self.connection.features.allows_auto_pk_0:
             raise ValueError('The database backend does not accept 0 as a '
                              'value for AutoField.')
         return value
@@ -266,6 +267,9 @@ class DatabaseOperations(BaseDatabaseOperations):
     def max_name_length(self):
         return 64
 
+    def pk_default_value(self):
+        return 'NULL'
+
     def bulk_insert_sql(self, fields, placeholder_rows):
         placeholder_rows_sql = (", ".join(row) for row in placeholder_rows)
         values_sql = ", ".join("(%s)" % sql for sql in placeholder_rows_sql)

tests/backends/mysql/test_features.py

@@ -32,3 +32,9 @@ class TestFeatures(TestCase):
             database_features = DatabaseFeatures(_connection)
             self.assertFalse(database_features.has_select_for_update_skip_locked)
             self.assertFalse(database_features.has_select_for_update_nowait)
+
+    def test_allows_auto_pk_0(self):
+        with mock.MagicMock() as _connection:
+            _connection.sql_mode = {'NO_AUTO_VALUE_ON_ZERO'}
+            database_features = DatabaseFeatures(_connection)
+            self.assertIs(database_features.allows_auto_pk_0, True)

tests/backends/tests.py

@@ -806,7 +806,8 @@ class ThreadTests(TransactionTestCase):
 class MySQLPKZeroTests(TestCase):
     """
     Zero as id for AutoField should raise exception in MySQL, because MySQL
-    does not allow zero for autoincrement primary key.
+    does not allow zero for autoincrement primary key if the
+    NO_AUTO_VALUE_ON_ZERO SQL mode is not enabled.
     """
     @skipIfDBFeature('allows_auto_pk_0')
     def test_zero_as_autoval(self):

tests/bulk_create/tests.py

@@ -115,7 +115,8 @@ class BulkCreateTests(TestCase):
     def test_zero_as_autoval(self):
         """
         Zero as id for AutoField should raise exception in MySQL, because MySQL
-        does not allow zero for automatic primary key.
+        does not allow zero for automatic primary key if the
+        NO_AUTO_VALUE_ON_ZERO SQL mode is not enabled.
         """
         valid_country = Country(name='Germany', iso_two_letter='DE')
         invalid_country = Country(id=0, name='Poland', iso_two_letter='PL')

tests/serializers/test_data.py

@@ -379,8 +379,8 @@ if connection.features.interprets_empty_strings_as_nulls:
                          data[3] is None)]
 
 # Regression test for #8651 -- a FK to an object with PK of 0
-# This won't work on MySQL since it won't let you create an object
-# with an autoincrement primary key of 0,
+# This won't work on MySQL without the NO_AUTO_VALUE_ON_ZERO SQL mode since it
+# won't let you create an object with an autoincrement primary key of 0.
 if connection.features.allows_auto_pk_0:
     test_data.extend([
         (data_obj, 0, Anchor, "Anchor 0"),