Step 2: Specify a data structure that maps between source and target databases using YAML

• By default, a Migrator uses the base Mapper and Mapping classes
• We can override this by supplying custom classes and configuration data:

---
# ...
      game_genre:
        target_key: game_type
        mapping:
          class: Class::DBI::DataMigration::Mapping::HasAToHasA
          config:
            target_class: New::DBI::GameType
            target_class_search_key: name
            matching_source_key: game_genre->game_genre_name