What is the proper way to eager load associated objects for a 'has_many' association?

Question

I am using Ruby on Rails 4 and I would like to eager load associated objects for a has_many association. That is, I have the following models:

class Article < ActiveRecord::Base
  has_many :comments
end

class Comment < ActiveRecord::Base
  belongs_to :article
end

Since I frequently retrieve comments when I load articles I would like to eager load comments each time I search for articles, for example in these cases:

# In these cases the eager loading should happen.
Article.find(1)
Article.where(:title => 'Sample text')
@current_user.articles

What is the proper way to handle this issue? How?

Marek Lipka · Accepted Answer · 2014-03-04T16:29:02.853

3

You should use includes method, like in following:

Article.includes(:comments).where(title: 'Sample text')

For your first case, you don't really need eager loading, since you only select one record, so this is not n+1 problem. You can just do:

article = Article.find(1)
article.comments

If you use include your articles frequently, you may define scope:

scope :with_comments, -> { includes(:comments }

and simply call it:

Article.with_comments.where(title: 'Sample text')

When your relation into which you want to include comments comes from another association, nothing changes, you can do both:

@current_user.articles.includes(:comments)

and

@current_user.articles.with_comments

edited Mar 04 '14 at 16:29

answered Mar 04 '14 at 16:08

Marek Lipka

50,622
7
87
91

Should I add the `includes` statement directly to the `has_many` association? – user502052 Mar 04 '14 at 16:10
@user502052 no, since it even won't do your job. – Marek Lipka Mar 04 '14 at 16:16
And how to solve the eager loading in cases when I run code as like: `@current_user.articles`? - Note: I updated the question since I forgot to add the mentioned case. – user502052 Mar 04 '14 at 16:26
@user502052 I edited my answer to cover your expanded question. – Marek Lipka Mar 04 '14 at 16:30

score 0 · Answer 2 · answered Mar 04 '14 at 16:11

0

If you want comments to be eager loaded automatically whenever you load article, you can use default_scope on Article model

class Article < ActiveRecord::Base
  has_many :comments
  default_scope :include => :comments
end

answered Mar 04 '14 at 16:11

usha

28,973
5
72
93

What is the proper way to eager load associated objects for a 'has_many' association?

2 Answers2