[4.1.x] Refs #34058 -- Fixed changing/deleting sequences when altering pre-Django 4.1 auto fields on PostgreSQL.

Thanks Anders Kaseorg for the report.

Follow up to 19e6efa50b603af325e7f62058364f278596758f.
Regression in 2eea361eff58dd98c409c5227064b901f41bd0d6.

Backport of bc3b8f152452ba0e41f28baa93c0bf8f39cddb09 from main

1 files changed, 36 insertions, 25 deletions

diff --git a/django/db/backends/postgresql/schema.py b/django/db/backends/postgresql/schema.py
index fa6e0a8ebf..d95d7557f7 100644
--- a/django/db/backends/postgresql/schema.py
+++ b/django/db/backends/postgresql/schema.py
@@ -112,6 +112,13 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
                 )
         return None
 
+    def _get_sequence_name(self, table, column):
+        with self.connection.cursor() as cursor:
+            for sequence in self.connection.introspection.get_sequences(cursor, table):
+                if sequence["column"] == column:
+                    return sequence["name"]
+        return None
+
     def _alter_column_type_sql(self, model, old_field, new_field, new_type):
         self.sql_alter_column_type = "ALTER COLUMN %(column)s TYPE %(type)s"
         # Cast when data type changed.
@@ -168,44 +175,48 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
                 }
             )
             column = strip_quotes(new_field.column)
-            sequence_name = "%s_%s_seq" % (table, column)
             fragment, _ = super()._alter_column_type_sql(
                 model, old_field, new_field, new_type
             )
-            return fragment, [
-                (
-                    # Drop the sequence if exists (Django 4.1+ identity columns
-                    # don't have it).
-                    self.sql_delete_sequence
-                    % {
-                        "sequence": self.quote_name(sequence_name),
-                    },
-                    [],
-                ),
-            ]
+            # Drop the sequence if exists (Django 4.1+ identity columns don't
+            # have it).
+            other_actions = []
+            if sequence_name := self._get_sequence_name(table, column):
+                other_actions = [
+                    (
+                        self.sql_delete_sequence
+                        % {
+                            "sequence": self.quote_name(sequence_name),
+                        },
+                        [],
+                    )
+                ]
+            return fragment, other_actions
         elif new_is_auto and old_is_auto and old_internal_type != new_internal_type:
             fragment, _ = super()._alter_column_type_sql(
                 model, old_field, new_field, new_type
             )
             column = strip_quotes(new_field.column)
-            sequence_name = f"{table}_{column}_seq"
             db_types = {
                 "AutoField": "integer",
                 "BigAutoField": "bigint",
                 "SmallAutoField": "smallint",
             }
-            return fragment, [
-                # Alter the sequence type if exists (Django 4.1+ identity
-                # columns don't have it).
-                (
-                    self.sql_alter_sequence_type
-                    % {
-                        "sequence": self.quote_name(sequence_name),
-                        "type": db_types[new_internal_type],
-                    },
-                    [],
-                ),
-            ]
+            # Alter the sequence type if exists (Django 4.1+ identity columns
+            # don't have it).
+            other_actions = []
+            if sequence_name := self._get_sequence_name(table, column):
+                other_actions = [
+                    (
+                        self.sql_alter_sequence_type
+                        % {
+                            "sequence": self.quote_name(sequence_name),
+                            "type": db_types[new_internal_type],
+                        },
+                        [],
+                    ),
+                ]
+            return fragment, other_actions
         else:
             return super()._alter_column_type_sql(model, old_field, new_field, new_type)