Hey yo! one class per kitty

Kitty students start to register courses. A student can have many courses and a course can contain many students. Students and courses are many to many relationship via registration.

class Kitty < ActiveRecord::BASE
  has_many :registration
  has_many :courses, :through => :registration
end

class Registration < ActiveRecord::BASE
  belongs_to :kitty
  belongs_to :course
end

class Course < ActiveRecord::BASE
  has_many :registration
  has_many :kitty, :though => :registration
end

Now, the problem is, how to make sure each kitty is not registered more once for a particular course.

To do this, use validates_uniqueness_of

Validate the uniqueness of a kitty student id (kitty_id) against a course_id:

class Registration < ActiveRecord::BASE
  belongs_to :kitty
  belongs_to :course

  validates_uniqueness_of :kitty_id, 
                          :scope => :course_id,
                          :message => "one kitty per course!"
end

Remember! when validating the uniqueness via has_many :through, use specific attribute name instead of model name.

validates_uniqueness_of :kitty_id, 
                        :scope => :course_id,
                        :message => "one kitty per course!"

not

validates_uniqueness_of :kittt, 
                        :scope => :course,
                        :message => "one kitty per course!"