Using join tables in ruby on rails

Lets say I have two databases: one for students and one for classes. I would like to be able to 'add' classes to a specific student and also be able to add students to a specific class. I assume I need to use a join table here but I am a little lost on how to use them. I would ultimately like to be able to do something like:

  @class.students.find(@student_id)

and this would tell me if the student is in the class or not. I know the relationshi开发者_运维知识库p between classes and students is 'has_many' and vice versa. Does doing 't.references :students' in the migrate files accomplish that? I tried adding that line to my migrate file and then tried finding something using the statement above and it gave me an error. I am new to RoR so I am not even sure what the best way to go about achieving this is. Any help is appreciated!

Everything true what @Jordan said, here the concrete steps to take:

1. Create a migration: rails g model CourseStudent creates a join model for the n:m relation, and the migration to the corresponding table.

2. Edit the migration file CreateCourseStudent so it contains the following:

   class CreateCourseStudent < ActiveRecord::Migration
     def change
       create_table :course_students do |t|
   
         # Your code comes here
         t.integer :student_id
         t.integer :course_id
   
         # Here comes the generated code 
         t.timestamps
       end
     end
   end
   

3. Run the migration: rake db:migrate. As a result, the join table should now exist in your database.

4. Add to the models the following code

   class Course < ActiveRecord::Base
     has_many :course_students
     has_many :students, :through => :course_students
   end
   
   class Student < ActiveRecord::Base
     has_many :course_students
     has_many :courses, :through => :course_students
   end
   
   class CourseStudent < ActiveRecord::Base
     belongs_to :student
     belongs_to :course
   end
   

You are now able to use the methods generated by the methods belongs_to and has_many:

• @course.students
• @student.courses

Try to find all the relevant facts and snippets in the Rails Guides, there you should find all information you need to get on track. Good luck!

This is an old question, but just in case anyone stumbles upon this like I did, you can now have the relationships has_and_belongs_to_many. So yes, you would create a join table:

create_join_table :students, :courses do |t|
  t.integer :student_id
  t.integer :course_id
end

And then in the models, you would say that a student has_and_belongs_to_many :courses And a course has_and_belongs_to_many :students. There is no need to make a third class called CourseStudent. This link has all this information

Yes, this is a many-to-many relationship (class has many students, student has many classes). For this you'll use a has_many :through relation. Take a look at the docs for ActiveRecord::Associations (Ctrl-F for "Association Join Models").

In a migration, t.references :students is how you would specify a belongs_to relation, as it just adds a student_id column (which can only accommodate one id, i.e. one student). A join model, however, will have two columns: student_id and class_id. (Incidentally, calling a model 'Class' in Ruby is asking for trouble. Might I suggest 'Course'?)