[1.7.x] Fixed #24015 -- Factorized create_index_sql expression

Backport of 6072f17d0 from master, with one test reinforced.
Thanks Tim Graham for the review.

django/db/backends/schema.py

@@ -322,14 +322,9 @@ class BaseDatabaseSchemaEditor(object):
                 ))
             self.execute(self._delete_constraint_sql(self.sql_delete_index, model, constraint_names[0]))
         # Created indexes
-        for fields in news.difference(olds):
-            columns = [model._meta.get_field_by_name(field)[0].column for field in fields]
-            self.execute(self.sql_create_index % {
-                "table": self.quote_name(model._meta.db_table),
-                "name": self._create_index_name(model, columns, suffix="_idx"),
-                "columns": ", ".join(self.quote_name(column) for column in columns),
-                "extra": "",
-            })
+        for field_names in news.difference(olds):
+            fields = [model._meta.get_field_by_name(field)[0] for field in field_names]
+            self.execute(self._create_index_sql(model, fields, suffix="_idx"))
 
     def alter_db_table(self, model, old_db_table, new_db_table):
         """
@@ -389,14 +384,7 @@ class BaseDatabaseSchemaEditor(object):
             self.execute(sql)
         # Add an index, if required
         if field.db_index and not field.unique:
-            self.deferred_sql.append(
-                self.sql_create_index % {
-                    "name": self._create_index_name(model, [field.column], suffix=""),
-                    "table": self.quote_name(model._meta.db_table),
-                    "columns": self.quote_name(field.column),
-                    "extra": "",
-                }
-            )
+            self.deferred_sql.append(self._create_index_sql(model, [field]))
         # Add any FK constraints later
         if field.rel 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"))
@@ -637,15 +625,10 @@ class BaseDatabaseSchemaEditor(object):
         if not old_field.unique and new_field.unique:
             self.execute(self._create_unique_sql(model, [new_field.column]))
         # Added an index?
-        if not old_field.db_index and new_field.db_index and not new_field.unique and not (not old_field.unique and new_field.unique):
-            self.execute(
-                self.sql_create_index % {
-                    "table": self.quote_name(model._meta.db_table),
-                    "name": self._create_index_name(model, [new_field.column], suffix="_uniq"),
-                    "columns": self.quote_name(new_field.column),
-                    "extra": "",
-                }
-            )
+        if (not old_field.db_index and new_field.db_index and
+                not new_field.unique and not
+                (not old_field.unique and new_field.unique)):
+            self.execute(self._create_index_sql(model, [new_field], suffix="_uniq"))
         # Type alteration on primary key? Then we need to alter the column
         # referring to us.
         rels_to_update = []

docs/releases/1.7.2.txt

@@ -155,3 +155,6 @@ Bugfixes
 * Ensured the app registry is fully populated when unpickling models. When an
   external script (like a queueing infrastructure) reloads pickled models, it
   could crash with an ``AppRegistryNotReady`` exception (:ticket:`24007`).
+
+* Added quoting to field indexes in the SQL generated by migrations to prevent
+  a crash when the index name requires it (:ticket:`#24015`).

tests/indexes/tests.py

@@ -46,8 +46,16 @@ class SchemaIndexesTests(TestCase):
     Test index handling by the db.backends.schema infrastructure.
     """
     def test_index_together(self):
-        index_sql = connection.schema_editor()._model_indexes_sql(Article)
+        editor = connection.schema_editor()
+        index_sql = editor._model_indexes_sql(Article)
         self.assertEqual(len(index_sql), 1)
+        # Ensure the index name is properly quoted
+        self.assertIn(
+            connection.ops.quote_name(
+                editor._create_index_name(Article, ['headline', 'pub_date'], suffix='_idx')
+            ),
+            index_sql[0]
+        )
 
     def test_index_together_single_list(self):
         # Test for using index_together with a single list (#22172)