alter_table(:external_id) do
      add_foreign_key([:subject_id], :subject, :key => :id)
      add_foreign_key([:accession_id], :accession, :key => :id)
      add_foreign_key([:archival_object_id], :archival_object, :key => :id)
      add_foreign_key([:collection_management_id], :collection_management, :key => :id)