only in patch2:
unchanged:
--- a/modules/field/field.crud.inc
+++ b/modules/field/field.crud.inc
@@ -244,9 +244,11 @@ function field_update_field($field) {
   // $prior_field may no longer be right.
   module_load_install($field['module']);
   $schema = (array) module_invoke($field['module'], 'field_schema', $field);
-  $schema += array('columns' => array(), 'indexes' => array());
+  $schema += array('columns' => array(), 'indexes' => array(), 'foreign keys' => array());
   // 'columns' are hardcoded in the field type.
   $field['columns'] = $schema['columns'];
+  // 'foreign keys' are hardcoded in the field type.
+  $field['foreign keys'] = $schema['foreign keys'];
   // 'indexes' can be both hardcoded in the field type, and specified in the
   // incoming $field definition.
   $field += array(