An event has many artists, and an artist has many events. The join of an artist and an event is called a performance. I want to add artists to an event.

This works except that I'm getting duplicate entries into my join table when creating a new event. This causes problems elsewhere.

event.rb

has_many :performances, :dependent => :destroy
has_many :artists, :through => :performances

accepts_nested_attributes_for :artists, :reject_if => proc {|a| a['name'].blank?}
accepts_nested_attributes_for :performances, :reject_if => proc { |a| a['artist_id'].blank? }, :allow_destroy => true

artist.rb

has_many :performances, :dependent => :destroy
has_many :artists, :through => :performances

performance.rb

belongs_to :artist
belongs_to :event

events_controller.rb

def new
  @event = Event.new
  @event.artists.build

  respond_to do |format|
    format.html # new.html.erb
    format.xml  { render :xml => @event }
  end
end

def create
  @event = Event.new(params[:event])

  respond_to do |format|
    if @event.save
      flash[:notice] = 'Event was successfully created.'
      format.html { redirect_to(admin_events_url) }
      format.xml  { render :xml => @event, :status => :created, :location => @event }
    else
      format.html { render :action => "new" }
      format.xml  { render :xml => @event.errors, :status => :unprocessable_entity }
    end
  end
end

output

Performance Create (0.2ms)   INSERT INTO `performances` (`event_id`, `artist_id`) VALUES(7, 19)
Performance Create (0.1ms)   INSERT INTO `performances` (`event_id`, `artist_id`) VALUES(7, 19)