Refs #27090 -- Added real database sequence introspection.

Thanks Mariusz Felisiak for the Oracle part and Tim Graham for the
review.

django/db/backends/base/introspection.py

@@ -113,9 +113,10 @@ class BaseDatabaseIntrospection:
         all apps.
         """
         from django.apps import apps
-        from django.db import models, router
+        from django.db import router
 
         sequence_list = []
+        cursor = self.connection.cursor()
 
         for app_config in apps.get_app_configs():
             for model in router.get_migratable_models(app_config, self.connection.alias):
@@ -123,19 +124,23 @@ class BaseDatabaseIntrospection:
                     continue
                 if model._meta.swapped:
                     continue
-                for f in model._meta.local_fields:
+                sequence_list.extend(self.get_sequences(cursor, model._meta.db_table, model._meta.local_fields))
-                    if isinstance(f, models.AutoField):
-                        sequence_list.append({'table': model._meta.db_table, 'column': f.column})
-                        break  # Only one AutoField is allowed per model, so don't bother continuing.
-
                 for f in model._meta.local_many_to_many:
                     # If this is an m2m using an intermediate table,
                     # we don't need to reset the sequence.
                     if f.remote_field.through is None:
-                        sequence_list.append({'table': f.m2m_db_table(), 'column': None})
+                        sequence = self.get_sequences(cursor, f.m2m_db_table())
-
+                        sequence_list.extend(sequence if sequence else [{'table': f.m2m_db_table(), 'column': None}])
         return sequence_list
 
+    def get_sequences(self, cursor, table_name, table_fields=()):
+        """
+        Return a list of introspected sequences for table_name. Each sequence
+        is a dict: {'table': <table_name>, 'column': <column_name>}. An optional
+        'name' key can be added if the backend supports named sequences.
+        """
+        raise NotImplementedError('subclasses of BaseDatabaseIntrospection may require a get_sequences() method')
+
     def get_key_columns(self, cursor, table_name):
         """
         Backends can override this to return a list of:

django/db/backends/mysql/introspection.py

@@ -105,6 +105,13 @@ class DatabaseIntrospection(BaseDatabaseIntrospection):
             )
         return fields
 
+    def get_sequences(self, cursor, table_name, table_fields=()):
+        for field_info in self.get_table_description(cursor, table_name):
+            if 'auto_increment' in field_info.extra:
+                # MySQL allows only one auto-increment column per table.
+                return [{'table': table_name, 'column': field_info.name}]
+        return []
+
     def get_relations(self, cursor, table_name):
         """
         Return a dictionary of {field_name: (field_name_other_table, other_table)}

django/db/backends/oracle/introspection.py

@@ -3,6 +3,7 @@ from collections import namedtuple
 
 import cx_Oracle
 
+from django.db import models
 from django.db.backends.base.introspection import (
     BaseDatabaseIntrospection, FieldInfo as BaseFieldInfo, TableInfo,
 )
@@ -98,6 +99,33 @@ class DatabaseIntrospection(BaseDatabaseIntrospection):
         """Table name comparison is case insensitive under Oracle."""
         return name.lower()
 
+    def get_sequences(self, cursor, table_name, table_fields=()):
+        cursor.execute("""
+            SELECT
+                user_tab_identity_cols.sequence_name,
+                user_tab_identity_cols.column_name
+            FROM
+                user_tab_identity_cols,
+                user_constraints,
+                user_cons_columns cols
+            WHERE
+                user_constraints.constraint_name = cols.constraint_name
+                AND user_constraints.table_name = user_tab_identity_cols.table_name
+                AND cols.column_name = user_tab_identity_cols.column_name
+                AND user_constraints.constraint_type = 'P'
+                AND user_tab_identity_cols.table_name = UPPER(%s)
+        """, [table_name])
+        # Oracle allows only one identity column per table.
+        row = cursor.fetchone()
+        if row:
+            return [{'name': row[0].lower(), 'table': table_name, 'column': row[1].lower()}]
+        # To keep backward compatibility for AutoFields that aren't Oracle
+        # identity columns.
+        for f in table_fields:
+            if isinstance(f, models.AutoField):
+                return [{'table': table_name, 'column': f.column}]
+        return []
+
     def get_relations(self, cursor, table_name):
         """
         Return a dictionary of {field_name: (field_name_other_table, other_table)}

django/db/backends/postgresql/introspection.py

@@ -82,6 +82,26 @@ class DatabaseIntrospection(BaseDatabaseIntrospection):
             for line in cursor.description
         ]
 
+    def get_sequences(self, cursor, table_name, table_fields=()):
+        sequences = []
+        cursor.execute("""
+            SELECT s.relname as sequence_name, col.attname
+            FROM pg_class s
+                JOIN pg_namespace sn ON sn.oid = s.relnamespace
+                JOIN pg_depend d ON d.refobjid = s.oid AND d.refclassid='pg_class'::regclass
+                JOIN pg_attrdef ad ON ad.oid = d.objid AND d.classid = 'pg_attrdef'::regclass
+                JOIN pg_attribute col ON col.attrelid = ad.adrelid AND col.attnum = ad.adnum
+                JOIN pg_class tbl ON tbl.oid = ad.adrelid
+                JOIN pg_namespace n ON n.oid = tbl.relnamespace
+            WHERE s.relkind = 'S'
+              AND d.deptype in ('a', 'n')
+              AND n.nspname = 'public'
+              AND tbl.relname = %s
+        """, [table_name])
+        for row in cursor.fetchall():
+            sequences.append({'name': row[0], 'table': table_name, 'column': row[1]})
+        return sequences
+
     def get_relations(self, cursor, table_name):
         """
         Return a dictionary of {field_name: (field_name_other_table, other_table)}

django/db/backends/sqlite3/introspection.py

@@ -85,6 +85,10 @@ class DatabaseIntrospection(BaseDatabaseIntrospection):
             ) for info in self._table_info(cursor, table_name)
         ]
 
+    def get_sequences(self, cursor, table_name, table_fields=()):
+        pk_col = self.get_primary_key_column(cursor, table_name)
+        return [{'table': table_name, 'column': pk_col}]
+
     def column_name_converter(self, name):
         """
         SQLite will in some cases, e.g. when returning columns from views and

docs/releases/2.0.txt

@@ -410,6 +410,10 @@ backends.
   for consistency). ``django.test`` also now passes those values as strings
   rather than as integers.
 
+* Third-party database backends should add a
+  ``DatabaseIntrospection.get_sequences()`` method based on the stub in
+  ``BaseDatabaseIntrospection``.
+
 Dropped support for Oracle 11.2
 -------------------------------
 

tests/backends/oracle/test_introspection.py

@@ -0,0 +1,29 @@
+import unittest
+
+from django.db import connection
+from django.test import TransactionTestCase
+
+from ..models import Person
+
+
+@unittest.skipUnless(connection.vendor == 'oracle', 'Oracle tests')
+class DatabaseSequenceTests(TransactionTestCase):
+    available_apps = []
+
+    def test_get_sequences(self):
+        with connection.cursor() as cursor:
+            seqs = connection.introspection.get_sequences(cursor, Person._meta.db_table, Person._meta.local_fields)
+            self.assertEqual(len(seqs), 1)
+            self.assertIsNotNone(seqs[0]['name'])
+            self.assertEqual(seqs[0]['table'], Person._meta.db_table)
+            self.assertEqual(seqs[0]['column'], 'id')
+
+    def test_get_sequences_manually_created_index(self):
+        with connection.cursor() as cursor:
+            with connection.schema_editor() as editor:
+                editor._drop_identity(Person._meta.db_table, 'id')
+                seqs = connection.introspection.get_sequences(cursor, Person._meta.db_table, Person._meta.local_fields)
+                self.assertEqual(seqs, [{'table': Person._meta.db_table, 'column': 'id'}])
+                # Recreate model, because adding identity is impossible.
+                editor.delete_model(Person)
+                editor.create_model(Person)

tests/backends/postgresql/test_introspection.py

@@ -0,0 +1,23 @@
+import unittest
+
+from django.db import connection
+from django.test import TestCase
+
+from ..models import Person
+
+
+@unittest.skipUnless(connection.vendor == 'postgresql', "Test only for PostgreSQL")
+class DatabaseSequenceTests(TestCase):
+    def test_get_sequences(self):
+        cursor = connection.cursor()
+        seqs = connection.introspection.get_sequences(cursor, Person._meta.db_table)
+        self.assertEqual(
+            seqs,
+            [{'table': Person._meta.db_table, 'column': 'id', 'name': 'backends_person_id_seq'}]
+        )
+        cursor.execute('ALTER SEQUENCE backends_person_id_seq RENAME TO pers_seq')
+        seqs = connection.introspection.get_sequences(cursor, Person._meta.db_table)
+        self.assertEqual(
+            seqs,
+            [{'table': Person._meta.db_table, 'column': 'id', 'name': 'pers_seq'}]
+        )

tests/introspection/tests.py

@@ -61,8 +61,9 @@ class IntrospectionTests(TransactionTestCase):
 
     def test_sequence_list(self):
         sequences = connection.introspection.sequence_list()
-        expected = {'table': Reporter._meta.db_table, 'column': 'id'}
+        reporter_seqs = [seq for seq in sequences if seq['table'] == Reporter._meta.db_table]
-        self.assertIn(expected, sequences, 'Reporter sequence not found in sequence_list()')
+        self.assertEqual(len(reporter_seqs), 1, 'Reporter sequence not found in sequence_list()')
+        self.assertEqual(reporter_seqs[0]['column'], 'id')
 
     def test_get_table_description_names(self):
         with connection.cursor() as cursor: