Postgres index names are limited to maximum length of 63 characters.
If index name is longer than 63 characters while running rails migration it throws error

Index name 'index_external_reservation_airport_transfers_on_external_reservation_id' on table 'external_reservation_airport_transfers' is too long; the limit is 63 characters

Fix is to explicitly specify the index name

t.references :external_reservation, null: false, foreign_key: true, index: {:name => 'idx_external_reservation_airport_transfers_external_reservation'}