CODEWITHSUNDEEP
ruby-on-railsactiverecordassociationshas-many-through
Eric
Eric

Reputation: 636

has_many :through with existing object

very similar to this question, but not quite the same.

I have Musician and Instrument models:

# musician.rb
class Musician < ActiveRecord::Base
  attr_accessible :instrument_attributes
  has_many :instrument_choices
  has_many :instruments, through: instrument_choices
  accepts_nested_attributes_for :instruments # with some other stuff
end

# instrument.rb
class Instrument < ActiveRecord::Base
  has_many :inverse_instrument_choices, class_name: "InstrumentChoice"
  has_many :musicians, through: :inverse_instrument_choices
  validates_uniqueness_of :name

# instrument_choice.rb
class InstrumentChoice < ActiveRecord::Base
  belongs_to :musician
  belongs_to :instrument
  validates_uniqueness_of :musician_id, scope: :instrument_id

I have a static list of possible instruments, and the user selects from that list in a select form in the new and edit views. Assume all these instruments have existing records. How do I handle adding new associations between musician and instrument?

Thanks!

Upvotes: 1

Views: 1051

Answers (1)

fphilipe
fphilipe

Reputation: 10054

You can create the InstrumentChoice either through an association or by creating the record directly:

musician.instrument_choices.create(instrument: an_instrument)
# or
InstrumentChoice.create(musician: a_musician, instrument: an_instrument)

Since you're not really storing any additional information in InstrumentChoice you could use a simple join-table that does not require its own model:

class Musician < ActiveRecord::Base
  has_and_belongs_to_many :instruments
end

class Instrument < ActiveRecord::Base
  has_and_belongs_to_many :musicians
end

musician.instruments << an_instrument

To ensure the uniqueness you can add a unique constraint to the join table.

Upvotes: 2

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