added join_column_index and inverse_join_column_index to object and table classes

source/core/object/attribute.cpp

@@ -32,6 +32,10 @@ std::string attribute::full_name() const {
   return owner ? owner->name() + "." + name_ : name_;
 }
 
+size_t attribute::index() const {
+  return index_;
+}
+
 const utils::field_attributes &attribute::attributes() const {
   return options_;
 }