Question

Sorry for the basic, basic question but I’m having some trouble understanding the RoR doc.

say I have 3 models–Students, Classes, and Enrolled_in. If it isn’t clear, students will enroll in classes, so Enrolled_in should have Students and Classes as foreign keys. I generated the models for each of these, but I’m confused what I should put into the associated migration file vs. the associated model file for each table. Do I specify the columns of the table in the migration file, and the key constraints in models?

If someone could clarify this, or tell me how they would solve the example question I posted, that would be really helpful. Thanks.

Answer 1

Firstly, to follow the convention the model name should be EnrolledIn, not Enrolled_in. Better yet, change it to Enrollment or something that has a definite meaning as a noun. And you’re also going to run into trouble trying to use Class (which is already a ruby object) as a model name. Perhaps change it to Course or something similar.

That aside, you should define all columns in your migrations. You can define the enrolled_ins table like this:

add_table :enrolled_ins do |t|
  t.references :student
  t.references :course        # I'm using course instead of class as noted above
end

The references shortcut will add a :student_id and :course_id as integer fields.

In your model files you’ll have:

# student.rb
class Student < ActiveRecord::Base
  has_many :enrolled_ins
  has_many :courses, :through => :enrolled_ins
end

# course.rb
class Course < ActiveRecord::Base
  has_many :enrolled_ins
  has_many :students, :through => :enrolled_ins
end

# enrolled_in.rb
class EnrolledIn < ActiveRecord::Base
  belongs_to :student
  belongs_to :course
end