Question

I am a beginner to rails framework. I have a fundamental question.

I am trying to define some models and their association referring the popular rails guides. My association looks like below.

class Person < ActiveRecord::Base
  has_one :head
end

class Head < ActiveRecord::Base
  belongs_to :person
end

Here, I need to add the foreign_key (Persons’s primary key) in the table ‘head’.

Now, if I need to get the ‘head’ of a ‘person’, rails need to scan through the head table and match the person_id.

The straight forward way I would think is to add the foreign key in ‘person’ table. Then I can directly refer the ‘head’ from ‘person’ with it’s ID.

It appears that rails convention is not performance friendly. Am I missing something here?

Answer 1

When you create the migration to add the column containing the foreign key, it is highly recommended to add an index on this column. This way, the database will efficiently find the Head from the person_id (as efficiently then a search by its id).

add_index :heads, :person_id

If it’s a one-to-one association, you can even add the unique option (unless your application accepts conjoined twins 🙂 ):

add_index :heads, :person_id, :unique => true

I suggest you to have a look to this 2 articles on where to use indexes:

• http://tomafro.net/2009/08/using-indexes-in-rails-index-your-associations
• http://tomafro.net/2009/08/using-indexes-in-rails-choosing-additional-indexes