One lookup table for two different joins

Hello Folks,

I'm trying to set up a Band model that uses one lookup table
(Instruments) to describe two things:

1. Instruments that the Band currently plays
2. Instruments that the Band needs

I only want to have one Instruments table to describe all instruments
for maintainability. Is there a way to do this? Here is my current
Band model:

class Band < ActiveRecord::Base
  attr_accessible :name, :description, :genre_ids, :instrument_ids

  # Band Instruments played
  has_many :band_plays_instruments
  has_many :instruments, :through => :band_plays_instruments

  # Band Instruments needed
  has_many :band_instrument_needs
  has_many :instruments, :through => :band_instrument_needs

end

The problem is that apparently I can't have two
"has_many :instruments, :through => x" statements in the model. Does
anybody have any recommendations as to how I should handle this?

Thank you in advance!

One table for instruments is the right idea, but you would probably be
better off then having separate join-tables for "instruments_wanted"
and "instuments_played". This would give you the functionality of
having extra attributes dedicated to those collections: an owned
instrument might have a record of which band members play it; desired
instruments might have a price that needs to be paid to buy it.

class Band < ActiveRecord::Base
       has_many :instruments_played
       has_many :instruments_needed
end

class InstrumentsPlayed < ActiveRecord::Base
  belongs_to :instrument
end

class InstrumentsNeeded < ActiveRecord::Base
  belongs_to :instrument
end

(of course, you might want to fiddle with the :class_name and table
names as these aren't intuitively pluralized - or just go with
"instruments_neededs" :slight_smile:

in your code you can now use:
  @band.instruments_played.each do |instrument_played|
    instrument_played.instrument ....

and
  @band.instruments_played << InstrumentPlayed.create(:instrument =>
Instrument.find_by_name("bassoon"))

Or you can add shortcuts to just the instruments (should you need) to
the band model:
  def band_plays_instruments
    instruments_played.inject([]) { |instrument_played| instruments <<
instrument_played.instrument }
  end