What is the send() method used for?

Question

Could someone please help me to understand what the 'send()' method listed below is used for? The code below, when I am reading it, makes no sense what purpose it's serving.

It's a Rails app using Ruby 1.8.7 with Rails 1.2.3. Please don't harp on me about upgrading, it's a client's environment, so I don't have that sort of leisure.

Needless to say though, the statement I am referring to is like this;

def do_schedule
  @performance = Performance.new(params[:performance])
  @performer = Performer.find(params[:performer_id])
  selected_track = params[:selected_track]
  if FileTest.exists?(File.expand_path(@performer.photo))
    @performance.photo = File.open(File.expand_path(@performer.photo))
  end

  @performance.audio = File.open(File.expand_path(@performer.send(selected_track)))

  if @performance.save
    flash[:notice] = 'Performer scheduled.'
    redirect_to :controller => :performer, :action => :index
  else
    render :action => 'schedule'
  end
end

Performer Model

class Performer < ActiveRecord::Base
  file_column :audio_one
  file_column :audio_two
  file_column :audio_three
  file_column :photo

  belongs_to :festival
  validates_presence_of :name, :first_name, :last_name, :address, :city, :state, :zip, :daytime_phone, :availability, :stages
  validates_format_of :email, :with => /\A([^@\s]+)@((?:[-a-z0-9]+\.)+[a-z]{2,})\Z/i
  validates_confirmation_of :email

  validates_presence_of :audio_one, :audio_two, :audio_three, :photo, :if => :submitted

  after_create :salt_access_key
  serialize :availability
  serialize :stages

  attr_accessor :other_type_of_music
  before_save :set_other_type

  def set_other_type
    if type_of_music == 'Other'
      self.type_of_music = "Other - #{other_type_of_music}" unless other_type_of_music.blank?
    end
  end

  def salt_access_key
    update_attribute(:access_key, Digest::SHA1.hexdigest("--#{self.id}--#{self.name}--#{self.festival.year}"))
  end

  def preferred_stages
    stages = []
    festival = Festival.find(self.festival_id.to_i)
    self.stages.collect { | key, value |
      id = key.gsub(/[\D]/, '').to_i
      if id > 0
        stages << festival.performance_stages.find(id).name
      end
    }
    return stages
  end
end

The controller that this is contained in is Performance. I have been scouring Google trying to figure out what purpose that '@performer.send(selected_track)' is actually doing, but feel like I'm rowing against a whirlpool.

Answer 1

The Ruby implementation for the send method, which is used to send a method message to an object, works like this:

class Car
  
  def start
    puts "vroom"
  end

  private

  def engine_temp
    puts "Just Right"
  end

end

@car = Car.new
@car.start # output: vroom
@car.send(:start) # output: vroom

That's the basics, an additional piece of important information is that send will allow you you send in messages to PRIVATE methods, not just public ones.

@car.engine_temp  # This doesn't work, it will raise an exception
@car.send(:engine_temp)  # output: Just Right

As for what your specific send call will do, more than likely there is a def method_missing in the Performer class that is setup to catch that and perform some action.

Answer 2

send is used to pass a method (and arguments) to an object. It's really handy when you don't know in advance the name of the method, because it's represented as a mere string or symbol.

Ex: Performer.find(params[:performer_id]) is the same as Performer.send(:find, params[:performer_id])

Beware here because relying on params when using send could be dangerous: what if users pass destroy or delete? It would actually delete your object.

Answer 3

The send method is the equivalent of calling the given method on the object. So if the selected_track variable has a value of 1234, then @performer.send(selected_track) is the same as @performer.1234. Or, if selected_track is "a_whiter_shade_of_pale" then it's like calling @performer.a_whiter_shade_of_pale.

Presumably, then, the Performer class overrides method_missing such that you can call it with any track (name or ID, it isn't clear from the above), and it will interpret that as a search for that track within that performer's tracks.