Ticket #34417 - Remove unnecessary recreation of foreign key constrains while dropping index.

django/db/backends/base/features.py

@@ -99,6 +99,10 @@ class BaseDatabaseFeatures:
     # Does the database have a copy of the zoneinfo database?
     has_zoneinfo_database = True
 
+    # Does the backend require the foreign key constraints to be recreated
+    # when dropping an index?
+    requires_fk_constraints_to_be_recreated = False
+
     # When performing a GROUP BY, is an ORDER BY NULL required
     # to remove any ordering?
     requires_explicit_null_ordering_when_grouping = False

django/db/backends/base/schema.py

@@ -919,6 +919,11 @@ def _alter_field(
         fks_dropped = set()
         if (
             self.connection.features.supports_foreign_keys
+            and (
+                self.connection.features.requires_fk_constraints_to_be_recreated
+                or not old_field.related_model == new_field.related_model
+                or getattr(new_field, "db_constraint", True) is False
+            )
             and old_field.remote_field
             and old_field.db_constraint
             and self._field_should_be_altered(

django/db/backends/mysql/features.py

@@ -15,6 +15,7 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     supports_regex_backreferencing = False
     supports_date_lookup_using_string = False
     supports_timezones = False
+    requires_fk_constraints_to_be_recreated = True
     requires_explicit_null_ordering_when_grouping = True
     atomic_transactions = False
     can_clone_databases = True

tests/schema/tests.py

@@ -1745,6 +1745,42 @@ class Meta:
             editor.alter_field(LocalBook, old_field, new_field, strict=True)
         self.assertForeignKeyExists(LocalBook, "author_id", "schema_author")
 
+    @skipUnlessDBFeature("supports_foreign_keys")
+    def test_fk_constraint_index_drop(self):
+        with connection.schema_editor() as editor:
+            editor.create_model(Author)
+            editor.create_model(Book)
+
+        author = Author.objects.create(name="Alice")
+        Book.objects.create(
+            author=author,
+            title="Much Ado About Foreign Keys",
+            pub_date=datetime.datetime.now(),
+        )
+
+        old_field = Book._meta.get_field("author")
+        new_field = ForeignKey(Author, CASCADE, db_index=False)
+        new_field.set_attributes_from_name("author")
+        with (
+            CaptureQueriesContext(connection) as ctx,
+            connection.schema_editor() as editor,
+        ):
+            editor.alter_field(Book, old_field, new_field, strict=True)
+
+        if connection.features.requires_fk_constraints_to_be_recreated:
+            self.assertTrue(
+                any(
+                    "DROP FOREIGN KEY" in query["sql"] for query in ctx.captured_queries
+                )
+            )
+            self.assertTrue(
+                any("ADD CONSTRAINT" in query["sql"] for query in ctx.captured_queries)
+            )
+        else:
+            self.assertFalse(
+                any("DROP CONSTRAINT" in query["sql"] for query in ctx.captured_queries)
+            )
+
     @skipUnlessDBFeature("supports_foreign_keys", "can_introspect_foreign_keys")
     def test_alter_o2o_to_fk(self):
         """