The Solution: YAML

Configure our custom Mapper in our YAML file:

---
# ... source and target connection, same as before
entities:
  TransGaming::Legacy::DBI::Games:
    target_cdbi_class: TransGaming::DBI::Game
    mapper:
      class: TransGaming::DataMigration::Legacy::Mapper::GamesToGame
    mappings:
    # ... mappings, same as before