The package line can be one of 3 things: entity, item or service, so I
would like to have a line type with the options entity, item or service.
I also want a field where I store the foreign key. The problem is that
the foreign key is dependent on what is selected in the line type field.
Before you attempt to solve such issues I would make sure you have a
good understanding of the basics of Rails, as you may well decide the
above is not the best way to solve the problem. I suggest you first
work right through a good tutorial such as railstutorial.org (which is
free to use online) including doing all the exercises. Then
reconsider your problem and come back with further questions if
necessary.