Fixed #30108 -- Allowed adding foreign key constraints in the same statement that adds a field.

django/db/backends/base/features.py

@@ -179,6 +179,9 @@ class BaseDatabaseFeatures:
     # Does it support foreign keys?
     supports_foreign_keys = True
 
+    # Can it create foreign key constraints inline when adding columns?
+    can_create_inline_fk = True
+
     # Does it support CHECK constraints?
     supports_column_check_constraints = True
     supports_table_check_constraints = True

django/db/backends/base/schema.py

@@ -77,6 +77,7 @@ class BaseDatabaseSchemaEditor:
         "REFERENCES %(to_table)s (%(to_column)s)%(deferrable)s"
     )
     sql_create_inline_fk = None
+    sql_create_column_inline_fk = None
     sql_delete_fk = sql_delete_constraint
 
     sql_create_index = "CREATE INDEX %(name)s ON %(table)s (%(columns)s)%(extra)s%(condition)s"
@@ -433,6 +434,22 @@ class BaseDatabaseSchemaEditor:
         db_params = field.db_parameters(connection=self.connection)
         if db_params['check']:
             definition += " " + self.sql_check_constraint % db_params
+        if field.remote_field and self.connection.features.supports_foreign_keys and field.db_constraint:
+            constraint_suffix = '_fk_%(to_table)s_%(to_column)s'
+            # Add FK constraint inline, if supported.
+            if self.sql_create_column_inline_fk:
+                to_table = field.remote_field.model._meta.db_table
+                to_column = field.remote_field.model._meta.get_field(field.remote_field.field_name).column
+                definition += " " + self.sql_create_column_inline_fk % {
+                    'name': self._fk_constraint_name(model, field, constraint_suffix),
+                    'column': self.quote_name(field.column),
+                    'to_table': self.quote_name(to_table),
+                    'to_column': self.quote_name(to_column),
+                    'deferrable': self.connection.ops.deferrable_sql()
+                }
+            # Otherwise, add FK constraints later.
+            else:
+                self.deferred_sql.append(self._create_fk_sql(model, field, constraint_suffix))
         # Build the SQL and run it
         sql = self.sql_create_column % {
             "table": self.quote_name(model._meta.db_table),
@@ -451,9 +468,6 @@ class BaseDatabaseSchemaEditor:
             self.execute(sql, params)
         # Add an index, if required
         self.deferred_sql.extend(self._field_indexes_sql(model, field))
-        # Add any FK constraints later
-        if field.remote_field and self.connection.features.supports_foreign_keys and field.db_constraint:
-            self.deferred_sql.append(self._create_fk_sql(model, field, "_fk_%(to_table)s_%(to_column)s"))
         # Reset connection if required
         if self.connection.features.connection_persists_old_columns:
             self.connection.close()
@@ -984,18 +998,8 @@ class BaseDatabaseSchemaEditor:
         }
 
     def _create_fk_sql(self, model, field, suffix):
-        def create_fk_name(*args, **kwargs):
-            return self.quote_name(self._create_index_name(*args, **kwargs))
-
         table = Table(model._meta.db_table, self.quote_name)
-        name = ForeignKeyName(
+        name = self._fk_constraint_name(model, field, suffix)
-            model._meta.db_table,
-            [field.column],
-            split_identifier(field.target_field.model._meta.db_table)[1],
-            [field.target_field.column],
-            suffix,
-            create_fk_name,
-        )
         column = Columns(model._meta.db_table, [field.column], self.quote_name)
         to_table = Table(field.target_field.model._meta.db_table, self.quote_name)
         to_column = Columns(field.target_field.model._meta.db_table, [field.target_field.column], self.quote_name)
@@ -1010,6 +1014,19 @@ class BaseDatabaseSchemaEditor:
             deferrable=deferrable,
         )
 
+    def _fk_constraint_name(self, model, field, suffix):
+        def create_fk_name(*args, **kwargs):
+            return self.quote_name(self._create_index_name(*args, **kwargs))
+
+        return ForeignKeyName(
+            model._meta.db_table,
+            [field.column],
+            split_identifier(field.target_field.model._meta.db_table)[1],
+            [field.target_field.column],
+            suffix,
+            create_fk_name,
+        )
+
     def _delete_fk_sql(self, model, name):
         return self._delete_constraint_sql(self.sql_delete_fk, model, name)
 

django/db/backends/mysql/schema.py

@@ -16,7 +16,10 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
     sql_rename_column = "ALTER TABLE %(table)s CHANGE %(old_column)s %(new_column)s %(type)s"
 
     sql_delete_unique = "ALTER TABLE %(table)s DROP INDEX %(name)s"
-
+    sql_create_column_inline_fk = (
+        ', ADD CONSTRAINT %(name)s FOREIGN KEY (%(column)s) '
+        'REFERENCES %(to_table)s(%(to_column)s)'
+    )
     sql_delete_fk = "ALTER TABLE %(table)s DROP FOREIGN KEY %(name)s"
 
     sql_delete_index = "DROP INDEX %(name)s ON %(table)s"

django/db/backends/oracle/schema.py

@@ -15,6 +15,7 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
     sql_alter_column_default = "MODIFY %(column)s DEFAULT %(default)s"
     sql_alter_column_no_default = "MODIFY %(column)s DEFAULT NULL"
     sql_delete_column = "ALTER TABLE %(table)s DROP COLUMN %(column)s"
+    sql_create_column_inline_fk = 'CONSTRAINT %(name)s REFERENCES %(to_table)s(%(to_column)s)%(deferrable)s'
     sql_delete_table = "DROP TABLE %(table)s CASCADE CONSTRAINTS"
     sql_create_index = "CREATE INDEX %(name)s ON %(table)s (%(columns)s)%(extra)s"
 

django/db/backends/postgresql/schema.py

@@ -15,6 +15,7 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
     sql_create_index = "CREATE INDEX %(name)s ON %(table)s%(using)s (%(columns)s)%(extra)s%(condition)s"
     sql_delete_index = "DROP INDEX IF EXISTS %(name)s"
 
+    sql_create_column_inline_fk = 'REFERENCES %(to_table)s(%(to_column)s)%(deferrable)s'
     # Setting the constraint to IMMEDIATE runs any deferred checks to allow
     # dropping it in the same transaction.
     sql_delete_fk = "SET CONSTRAINTS %(name)s IMMEDIATE; ALTER TABLE %(table)s DROP CONSTRAINT %(name)s"

django/db/backends/sqlite3/features.py

@@ -26,6 +26,7 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     atomic_transactions = False
     can_rollback_ddl = True
     supports_atomic_references_rename = Database.sqlite_version_info >= (3, 26, 0)
+    can_create_inline_fk = False
     supports_paramstyle_pyformat = False
     supports_sequence_reset = False
     can_clone_databases = True

docs/releases/3.0.txt

@@ -214,6 +214,10 @@ backends.
 
 * ``DatabaseIntrospection.get_field_type()`` may no longer return tuples.
 
+* If the database can create foreign keys in the same SQL statement that adds a
+  field, add ``SchemaEditor.sql_create_column_inline_fk`` with the appropriate
+  SQL; otherwise, set ``DatabaseFeatures.can_create_inline_fk = False``.
+
 Miscellaneous
 -------------
 

tests/indexes/tests.py

@@ -226,11 +226,9 @@ class SchemaIndexesMySQLTests(TransactionTestCase):
                 new_field.set_attributes_from_name('new_foreign_key')
                 editor.add_field(ArticleTranslation, new_field)
                 field_created = True
-                self.assertEqual([str(statement) for statement in editor.deferred_sql], [
+                # No deferred SQL. The FK constraint is included in the
-                    'ALTER TABLE `indexes_articletranslation` '
+                # statement to add the field.
-                    'ADD CONSTRAINT `indexes_articletrans_new_foreign_key_id_d27a9146_fk_indexes_a` '
+                self.assertFalse(editor.deferred_sql)
-                    'FOREIGN KEY (`new_foreign_key_id`) REFERENCES `indexes_article` (`id`)'
-                ])
         finally:
             if field_created:
                 with connection.schema_editor() as editor:

tests/schema/tests.py

@@ -241,6 +241,27 @@ class SchemaTests(TransactionTestCase):
             editor.alter_field(Book, old_field, new_field, strict=True)
         self.assertForeignKeyExists(Book, 'author_id', 'schema_tag')
 
+    @skipUnlessDBFeature('can_create_inline_fk')
+    def test_inline_fk(self):
+        # Create some tables.
+        with connection.schema_editor() as editor:
+            editor.create_model(Author)
+            editor.create_model(Book)
+            editor.create_model(Note)
+        self.assertForeignKeyNotExists(Note, 'book_id', 'schema_book')
+        # Add a foreign key from one to the other.
+        with connection.schema_editor() as editor:
+            new_field = ForeignKey(Book, CASCADE)
+            new_field.set_attributes_from_name('book')
+            editor.add_field(Note, new_field)
+        self.assertForeignKeyExists(Note, 'book_id', 'schema_book')
+        # Creating a FK field with a constraint uses a single statement without
+        # a deferred ALTER TABLE.
+        self.assertFalse([
+            sql for sql in (str(statement) for statement in editor.deferred_sql)
+            if sql.startswith('ALTER TABLE') and 'ADD CONSTRAINT' in sql
+        ])
+
     @skipUnlessDBFeature('supports_foreign_keys')
     def test_char_field_with_db_index_to_fk(self):
         # Create the table