Proper Rails Association to use

Question

I am trying to create an association between two tables. A student table and a computer table. A computer can only ever be assigned to one student (at any one time) but a student can be assigned to multiple computers.

This is what I currently have in mind. Setting up a has-many through relationship and modifying it a bit.

class Student < ActiveRecord::Base
  has_many :assignemnts
  has_many :computers, :through => :assignments
end

class Computer < ActiveRecord::Base
 has_one :assignment
 has_one :student, :through => :assignments
end

class Assignment < ActiveRecord::Base
  belongs_to :student
  belongs_to :computer
end

Does this seem like the best way to handle this problem? Or something better sound out quickly to the experts here. Thanks!

score 1 · Accepted Answer · answered May 29 '12 at 16:05

You need first to decide if a simple one-to many relationship is enough for you.

If yes, it gets a lot easier, because you can get rid of the Assignment-class and table.

Your database-table "computers" then needs a student_id column, with a non-unique index

Your models should look like this:

class Computer < ActiveRecord::Base
    belongs_to :student
end

class Student < ActiveRecord::Base
    has_many :computers, :dependent => :nullify
end

"dependent nullify" because you don't want to delete a computer when a student is deleted, but instead mark it as free.

Each of your computers can only be assigned to a single student, but you can reassign it to a different student, for example in the next year.

this actually seems like the best solution, it made the code look cleaner. — Dan, May 30 '12 at 12:14

alexkv · Answer 2 · 2012-05-30T06:31:17.057

0

You can also use has_and_belongs_to_many method. In your case it will be:

class Student < ActiveRecord::Base
    has_many :assignemnts
    has_and_belongs_to_many :computers, :join_table => 'assignments',
end

class Computer < ActiveRecord::Base
    has_one :assignment
    has_and_belongs_to_many :student, :join_table => 'assignments',
end

or you can rename assignments table to computers_students and remove join_table

class Student < ActiveRecord::Base
    has_many :assignemnts
    has_and_belongs_to_many :computers
end

class Computer < ActiveRecord::Base
    has_one :assignment
    has_and_belongs_to_many :student
end

edited May 30 '12 at 06:31

answered May 29 '12 at 13:26

alexkv

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i think table name should be computers_students – Dev R May 29 '12 at 13:45
Yes, you have to follow lexical order of the class names. – thesis May 29 '12 at 13:49

score 0 · Answer 3 · answered May 29 '12 at 13:55

Actually your approach is fine, as one offered by @alexkv. It is more discussion, than question.

Another thing if you want to use mapping table for some other purposes, like storing additional fields - then your approach is the best thing. In has_many :through table for the join model has a primary key and can contain attributes just like any other model.

From api.rubyonrails.org:

Choosing which way to build a many-to-many relationship is not always simple. If you need to work with the relationship model as its own entity, use has_many :through. Use has_and_belongs_to_many when working with legacy schemas or when you never work directly with the relationship itself.

I can advise you read this, to understand what approach better to choose in your situation:

Proper Rails Association to use

3 Answers3