Eager Loading

The N+1 Queries issue

NoBrainer allows eager loading of associations with eager_load() to avoid the N+1 queries issue. Suppose we have posts and comments:

class Author
  include NoBrainer::Document
  has_many :posts
end

class Post
  include NoBrainer::Document
  belongs_to :author
  has_many :comments
end

class Comment
  include NoBrainer::Document
  belongs_to :post
end

Suppose that we want to display the list of the most recent comments along with their post and author. The following will trigger 201 database queries:

comments = Comment.order_by(:created_at => :desc).limit(100)
comments.each do |comment|
  puts comment.post.author
end

Using eager_load() in a query

We can use eager_load to eager load the models to reduce the number of queries to 3:

comments = Comment.eager_load(:post => :author)
                  .order_by(:created_at => :desc).limit(100)
comments.each do |comment|
  puts comment.post.author
end

Post.eager_load(:author, :comments).each do |post|
  post.author
  post.comments.each { ... }
end

eager_load() accepts arrays and hashes to describe the associations to eager load. For example:

Model1.eager_load(:model2, :model3 => [:model4, :model5 => :model6])

NoBrainer allows criteria to be specified on how to include these associations. When specifying criteria, nested eager_load can be used to further load associations. For example:

Author.eager_load(:posts => Post.where(:body => /rethinkdb/)
                    .eager_load(:comments => Comment.order_by(:created_at)))

Remember that NoBrainer will use the model default scopes on all associations, except on the belongs_to associations.

NoBrainer uses in queries such as Model.where(foreign_key.in => [ids]) to retrieve associations and can leverage secondary indexes. You may add an :index => true option on the belongs_to association to improve read performance (at the cost of write performance).

NoBrainer prefers such queries because RethinkDB outer joins cannot be used with an index, and also joins often retrieve too much data when instances are shared across associations.

Using eager_load on an array

NoBrainer allows to eager loading associations on a given array of model instances with NoBrainer.eager_load(). The first argument is the array of model instances, and the second argument is the same as the criteria argument. For example:

posts = Post.where(...).to_a
NoBrainer.eager_load(posts, :comments)