gistfile1.rb

I want to create Task management app. Task has many assignee and one assignor. This is how I have created a association. Getting error of 
ActiveRecord::HasManyThroughAssociationNotFoundError: Could not find the association :assignment in model User


class User < ActiveRecord::Base
  attr_accessible :name
  has_many :assignments,:foreign_key => "assignee_id"
  has_many :tasks, :through => :assignment
end

class Task < ActiveRecord::Base
  attr_accessible :name
  has_many :assignments
end

class Assignment < ActiveRecord::Base
  attr_accessible :task_id ,:assignee_id
  belongs_to :assignee , :class_name => "User",:foreign_key => "user_id"
  belongs_to :task
end