Fixed #14180 -- Prevented unneeded index creation on MySQL-InnoDB

Thanks zimnyx for the report and Simon Charette, Tim Graham for
the reviews.

2 files changed, 21 insertions, 7 deletions

diff --git a/tests/commands_sql/tests.py b/tests/commands_sql/tests.py
index e1d4272616..10f0404776 100644
--- a/tests/commands_sql/tests.py
+++ b/tests/commands_sql/tests.py
@@ -72,14 +72,14 @@ class SQLCommandsTestCase(TestCase):
         with warnings.catch_warnings():
             warnings.simplefilter("ignore", category=RemovedInDjango20Warning)
             output = sql_indexes(app_config, no_style(), connections[DEFAULT_DB_ALIAS])
-        # PostgreSQL creates one additional index for CharField
-        self.assertIn(self.count_ddl(output, 'CREATE INDEX'), [3, 4])
+        # Number of indexes is backend-dependent
+        self.assertTrue(1 <= self.count_ddl(output, 'CREATE INDEX') <= 4)
 
     def test_sql_destroy_indexes(self):
         app_config = apps.get_app_config('commands_sql')
         output = sql_destroy_indexes(app_config, no_style(), connections[DEFAULT_DB_ALIAS])
-        # PostgreSQL creates one additional index for CharField
-        self.assertIn(self.count_ddl(output, 'DROP INDEX'), [3, 4])
+        # Number of indexes is backend-dependent
+        self.assertTrue(1 <= self.count_ddl(output, 'DROP INDEX') <= 4)
 
     def test_sql_all(self):
         app_config = apps.get_app_config('commands_sql')
@@ -88,8 +88,8 @@ class SQLCommandsTestCase(TestCase):
             output = sql_all(app_config, no_style(), connections[DEFAULT_DB_ALIAS])
 
         self.assertEqual(self.count_ddl(output, 'CREATE TABLE'), 3)
-        # PostgreSQL creates one additional index for CharField
-        self.assertIn(self.count_ddl(output, 'CREATE INDEX'), [3, 4])
+        # Number of indexes is backend-dependent
+        self.assertTrue(1 <= self.count_ddl(output, 'CREATE INDEX') <= 4)
 
 
 class TestRouter(object):
diff --git a/tests/indexes/tests.py b/tests/indexes/tests.py
index 86000bb94c..0710446245 100644
--- a/tests/indexes/tests.py
+++ b/tests/indexes/tests.py
@@ -5,7 +5,7 @@ from django.db import connection
 from django.test import TestCase
 from django.test.utils import IgnorePendingDeprecationWarningsMixin
 
-from .models import Article, IndexTogetherSingleList
+from .models import Article, ArticleTranslation, IndexTogetherSingleList
 
 
 class CreationIndexesTests(IgnorePendingDeprecationWarningsMixin, TestCase):
@@ -82,3 +82,17 @@ class SchemaIndexesTests(TestCase):
         """Test indexes are not created for related objects"""
         index_sql = connection.schema_editor()._model_indexes_sql(Article)
         self.assertEqual(len(index_sql), 1)
+
+    @skipUnless(connection.vendor == 'mysql', "This is a mysql-specific issue")
+    def test_no_index_for_foreignkey(self):
+        """
+        MySQL on InnoDB already creates indexes automatically for foreign keys.
+        (#14180).
+        """
+        storage = connection.introspection.get_storage_engine(
+            connection.cursor(), ArticleTranslation._meta.db_table
+        )
+        if storage != "InnoDB":
+            self.skip("This test only applies to the InnoDB storage engine")
+        index_sql = connection.schema_editor()._model_indexes_sql(ArticleTranslation)
+        self.assertEqual(index_sql, [])