Specific preloading with scoped associations example

This is an example of using scoped associations with Active Record, from Ruby on Rails, to preload already filtered associations.

Like this...

class Post < ActiveRecord::Base
  has_many :comments

  # Here you are defining an association, that will use Comment.popular,
  # to filter the Comment records.
  has_many :popular_comments, -> { popular }, class_name: "Comment"
end

class Comment < ActiveRecord::Base
  POPULAR = 9

  belongs_to :post

  # Here we are definining what `popular` means.
  scope :popular, -> { where(likes_count: POPULAR..) }
end

# Then you an preload directly just the popular comments
posts = Post.preload(:popular_comments).limit(5)
# => SELECT "posts".* FROM "posts" LIMIT $1  [["LIMIT", 5]]
# => SELECT "comments".* FROM "comments" WHERE "comments"."likes_count" >= $1 AND "comments"."post_id" IN ($1, $2, $3, $4, $5)

posts.each do |post|
  render_title(post)

  # You can call the association for each post without n+1 queries
  # because the association has already been preloaded
  post.popular_comments.each do |comment|
    render_comment(comment)
  end
end

In this way you can use your defined scopes, to build "scoped" associations to help you preload just the records that your are going to need and save some memory.

How to run the examples

Install the dependencies with bundle install.
Database setup - run the command:

ruby db/setup.rb

Run the examples with ruby examples/<file name>. For example:

ruby example/00_without_scoped_association.rb

Change the seeds on db/seeds.rb and re-run ruby db/setup.rb to test different scenarios.

This example uses the Active Record Playground by bhserna

bhserna/specific_preloading_with_scoped_associations

Specific preloading with scoped associations example

How to run the examples