The sequel gem is a database toolkit that allows you to interact with most databases. PostgreSQL has support for composite primary keys, but sequel, which is supposed to return the id of newly inserted records, isn’t sure what to return when faced with a composite primary key. You can get around this by telling sequel exactly what should be returned using the #returning method. For instance, get it to return just the id of the new record:

DB[:floors].returning(:id).insert(hotel_id: 4, id: 1, ...)
# [{id: 1}]

To get it to return both parts of composite key:

DB[:floors].returning(:id, :hotel_id).insert(hotel_id: 4, id: 1, ...)
# [{id: 1, hotel_id: 4}]