Refs #31335 -- Added SchemaEditor._create_missing_fk_index() on MySQL.

django/db/backends/mysql/schema.py

@@ -135,24 +135,35 @@ def _field_should_be_indexed(self, model, field):
             return False
         return not self._is_limited_data_type(field)
 
-    def _delete_composed_index(self, model, fields, *args):
+    def _create_missing_fk_index(
+        self,
+        model,
+        *,
+        fields,
+    ):
         """
         MySQL can remove an implicit FK index on a field when that field is
         covered by another index like a unique_together. "covered" here means
-        that the more complex index starts like the simpler one.
-        https://bugs.mysql.com/bug.php?id=37910 / Django ticket #24757
-        We check here before removing the [unique|index]_together if we have to
-        recreate a FK index.
+        that the more complex index has the FK field as its first field (see
+        https://bugs.mysql.com/bug.php?id=37910).
+
+        Manually create an implicit FK index to make it possible to remove the
+        composed index.
         """
         first_field = model._meta.get_field(fields[0])
         if first_field.get_internal_type() == "ForeignKey":
             constraint_names = self._constraint_names(
-                model, [first_field.column], index=True
+                model,
+                [first_field.column],
+                index=True,
             )
             if not constraint_names:
                 self.execute(
                     self._create_index_sql(model, fields=[first_field], suffix="")
                 )
+
+    def _delete_composed_index(self, model, fields, *args):
+        self._create_missing_fk_index(model, fields=fields)
         return super()._delete_composed_index(model, fields, *args)
 
     def _set_field_new_type_null_status(self, field, new_type):