Validates uniqueness from foreign key of foreign key

I have 3 models in my rails application, User, Course, and CourseTemplate.

A Course belongs to a User and a CourseTemplate belongs to a Course.

What I want to do is to validate the uniqueness between the CourseTemplate name and the User id.

Is this possible?

Without denormalization of data

class CourseTemplate < ActiveRecord::Base
  belongs_to :course
  has_one :user, through: :course
  validate :unique_course_template_for_user

  private

  def unique_course_template_for_user
    errors.add(:name, 'Must be unique') if CourseTemplate.find_by(user: user.id, name: self.name).count > 0
  end
end

With denormalization of data

If you're ok with some denormalization of your data, you could add user_id to CourseTemplate, and then simply use the scope feature of validates uniqueness.

Below I show how to use callbacks to maintain the user_id in the CourseTemplate. Note that it assumes a course cannot be moved to a different user.

class CourseTemplate < ActiveRecord::Base
  before_create :copy_user_id_from_course

  validates :name, uniqueness: { scope: :user_id, message: 'Must be unique for the same user'}

  private

  def copy_user_id_from_course
    self.user_id = course.user_id
    true
  end
end

If the course can be moved to a different user, you should add a callback on Course:

class Course < ActiveRecord::Base
  after_save :set_course_templates_user, if: :user_id_changed?

  private

  def set_course_templates_user
    course_templates.update_all user_id: self.user_id
  end

end