Fixed #24757 -- Reverted "Prevented unneeded index creation on MySQL-…

…InnoDB"

This reverts commit 2ceb10f (refs #14180).

django/db/backends/mysql/schema.py

@@ -50,18 +50,6 @@ def add_field(self, model, field):
                 'column': self.quote_name(field.column),
             }, [effective_default])
 
-    def _model_indexes_sql(self, model):
-        storage = self.connection.introspection.get_storage_engine(
-            self.connection.cursor(), model._meta.db_table
-        )
-        if storage == "InnoDB":
-            for field in model._meta.local_fields:
-                if field.db_index and not field.unique and field.get_internal_type() == "ForeignKey":
-                    # Temporary setting db_index to False (in memory) to disable
-                    # index creation for FKs (index automatically created by MySQL)
-                    field.db_index = False
-        return super(DatabaseSchemaEditor, self)._model_indexes_sql(model)
-
     def _alter_column_type_sql(self, table, old_field, new_field, new_type):
         # Keep null property of old field, if it has changed, it will be handled separately
         if old_field.null:

docs/releases/1.8.2.txt

@@ -25,3 +25,6 @@ Bugfixes
   pointing to :class:`~django.db.models.UUIDField` and inheritance on models
   with ``UUIDField`` primary keys work correctly (:ticket:`24698`,
   :ticket:`24712`).
+
+* Added back MySQL index creation for foreign keys when using the InnoDB
+  storage engine due to problems when dropping other indexes (:ticket:`24757`).

docs/releases/1.8.txt

@@ -286,6 +286,8 @@ Database backends
 
 * The MySQL backend no longer creates explicit indexes for foreign keys when
   using the InnoDB storage engine, as MySQL already creates them automatically.
+  This change was reverted in :doc:`1.8.2 </releases/1.8.2>` because it
+  created problems when dropping other indexes without this index.
 
 * The Oracle backend no longer defines the ``connection_persists_old_columns``
   feature as ``True``. Instead, Oracle will now include a cache busting clause

tests/indexes/tests.py

@@ -3,7 +3,7 @@
 from django.db import connection
 from django.test import TestCase
 
-from .models import Article, ArticleTranslation, IndexTogetherSingleList
+from .models import Article, IndexTogetherSingleList
 
 
 class SchemaIndexesTests(TestCase):
@@ -59,17 +59,3 @@ def test_postgresql_virtual_relation_indexes(self):
         """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, [])