how to validate associated model id?

Question

I have a student and a course model. Student belongs to course, and course has many students.

class Student < ActiveRecord::Base
  attr_accessible :course_id,         


        

Answer 1

You should look into creating a custom validation method:

class Student < ActiveRecord::Base
  validates :course_id, presence: true, numericality: { only_integer: true }
  ...
  validate :validate_course_id

private

  def validate_course_id
    errors.add(:course_id, "is invalid") unless Course.exists?(self.course_id)
  end
end

First, your model will make sure that the course_id is a valid integer, and then your custom validation will make sure that the course exists in the database.

Answer 2

You can validate the presence of the course association. So, if does not exists, valid? will be false.

Your class should be:

class Student < ActiveRecord::Base
  attr_accessible :course_id, :name, :password, :status, :studentID, :year
  belongs_to :course

  validates :course, :name, :password, :status, :studentID, :year, presence: true
end

The differences of validating foreign key and the association itself are explained at this post.

If you don't need to validate presence of course, you can do this workaround:

validates :course, presence: true, if: -> {course_id.present?}

Answer 3

You can just do a custom validation method that checks the validity of your course id:

# course.rb
validates :course_id_is_valid

def course_id_is_valid
  if #course id isn't valid
    errors.add(:course_id, "is invalid")
  end
end

Alternately, if the course list is a discrete list, you can use the :in option on validations:

# course.rb
validates :course_id, :inclusion => { :in => list_of_courses, :message => "%{value} is not a course id" }

There are more options. Both of these examples come from the Rails docs:

http://guides.rubyonrails.org/active_record_validations_callbacks.html#custom-methods