Merge branch 'azure-1.11' into azure-2.0

Author: michiya

sql_server/pyodbc/schema.py

@@ -134,6 +134,12 @@ def _alter_field(self, model, old_field, new_field, old_type, new_type,
         # Drop any FK constraints, we'll remake them later
         fks_dropped = set()
         if old_field.remote_field and old_field.db_constraint:
+            # Drop index, SQL Server requires explicit deletion
+            if not new_field.db_constraint:
+                index_names = self._constraint_names(model, [old_field.column], index=True)
+                for index_name in index_names:
+                    self.execute(self._delete_constraint_sql(self.sql_delete_index, model, index_name))
+
             fk_names = self._constraint_names(model, [old_field.column], foreign_key=True)
             if strict and len(fk_names) != 1:
                 raise ValueError("Found wrong number (%s) of foreign key constraints for %s.%s" % (