Fixed #26171 -- Made MySQL create an index on ForeignKeys with db_contraint=False.

Refactored "Prevented unneeded index creation on MySQL-InnoDB" (2ceb10f)
to avoid setting db_index=False.

1 files changed, 11 insertions, 8 deletions

diff --git a/django/db/backends/mysql/schema.py b/django/db/backends/mysql/schema.py
index 802ce6353b..4aa40f7cb5 100644
--- a/django/db/backends/mysql/schema.py
+++ b/django/db/backends/mysql/schema.py
@@ -51,17 +51,20 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
                 'column': self.quote_name(field.column),
             }, [effective_default])
 
-    def _model_indexes_sql(self, model):
+    def _field_should_be_indexed(self, model, field):
+        create_index = super(DatabaseSchemaEditor, self)._field_should_be_indexed(model, field)
         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)
+        # No need to create an index for ForeignKey fields except if
+        # db_constraint=False because the index from that constraint won't be
+        # created.
+        if (storage == "InnoDB" and
+                create_index and
+                field.get_internal_type() == 'ForeignKey' and
+                field.db_constraint):
+            return False
+        return create_index
 
     def _delete_composed_index(self, model, fields, *args):
         """