Rails, has_many :through, undefined method `to_sym` for nil:NilClass

Question

I have been learning rails and i have stumbled one problem with relationships. I have many-to-many association User - Tournament, and for some reason i can't access participated_tournaments on User instances, or can't access participants on Tournament instances.

2.0.0-p643 :001 > Tournament.new.participants
NoMethodError: undefined method `to_sym' for nil:NilClass
from /home/marcin/.rvm/gems/ruby-2.0.0-p643/gems/activerecord-4.1.8/lib/active_record/reflection.rb:100:in `_reflect_on_association'
from /home/marcin/.rvm/gems/ruby-2.0.0-p643/gems/activerecord-4.1.8/lib/active_record/reflection.rb:537:in `source_reflection'

User model

class User < ActiveRecord::Base
  has_many :participants_tournaments
  has_many :participated_tournaments, :through => :participants_tournaments
end

Tournament model

class Tournament < ActiveRecord::Base
  has_many :participants_tournaments
  has_many :participants, :through => :participants_tournaments
end

ParticipantsTournament model

class ParticipantsTournament < ActiveRecord::Base
  belongs_to :tournament
  belongs_to :user
end

ParticipantsTournament migration

def change
  create_table :participants_tournaments, :id => false do |t|
    t.integer "tournament_id",
    t.integer "user_id"
  end
end

I have read: This topic on SO, also this topic and watched/read this railscast, but i can't seem to finally get it working.

Answer 1

When you are calling .new on tournaments it does not have an id, so it will be nil. Therefore will not find the user.

Also when you are remapping a model in a has many through like this you have to tell active record what the model it is looking for is. You do this with the :source argument. Here is how tournament would work.

class Tournament < ActiveRecord::Base
  has_many :participants_tournaments
  has_many :participants, :through => :participants_tournaments, :source => :user
end