Goldiloader

Wouldn't it be awesome if ActiveRecord didn't make you think about eager loading and it just did the "right" thing by default? With Goldiloader it can!

This branch only supports Rails 5.2+ with Ruby 2.6+. For older versions of Rails/Ruby use 3-x-stable, 2-x-stable or 1-x-stable.

Consider the following models:

class Blog < ActiveRecord::Base
  has_many :posts
end

class Post < ActiveRecord::Base
  belongs_to :blog
end

Here are some sample queries without the Goldiloader:

> blogs = Blogs.limit(5).to_a
# SELECT * FROM blogs LIMIT 5

> blogs.each { |blog| blog.posts.to_a }
# SELECT * FROM posts WHERE blog_id = 1
# SELECT * FROM posts WHERE blog_id = 2
# SELECT * FROM posts WHERE blog_id = 3
# SELECT * FROM posts WHERE blog_id = 4
# SELECT * FROM posts WHERE blog_id = 5

Here are the same queries with the Goldiloader:

> blogs = Blogs.limit(5).to_a
# SELECT * FROM blogs LIMIT 5

> blogs.each { |blog| blog.posts.to_a }
# SELECT * FROM posts WHERE blog_id IN (1,2,3,4,5)

Whoa! It automatically loaded all of the posts for our five blogs in a single database query without specifying any eager loads! Goldiloader assumes that you'll access all models loaded from a query in a uniform way. The first time you traverse an association on any of the models it will eager load the association for all the models. It even works with arbitrary nesting of associations.

Read more about the motivation for the Goldiloader in this blog post.

Installation

Add this line to your application's Gemfile:

gem 'goldiloader'

And then execute:

$ bundle

Or install it yourself as:

$ gem install goldiloader

Usage

By default all associations will be automatically eager loaded when they are first accessed so hopefully most use cases should require no additional configuration. Note you're still free to explicitly eager load associations via eager_load, includes, or preload.

Disabling Automatic Eager Loading

You can disable automatic eager loading with auto_include query scope method:

Blog.order(:name).auto_include(false)

This can also be used to disable automatic eager loading for associations:

class Blog < ActiveRecord::Base
  has_many :posts, -> { auto_include(false) }
end

Association Options

Goldiloader supports a few options on ActiveRecord associations to customize its behavior.

fully_load

There are several association methods that ActiveRecord can either execute on in memory models or push down into SQL depending on whether or not the association is loaded. This includes the following methods:

• first
• second
• third
• fourth
• fifth
• forty_two (one of the hidden gems in Rails 4.1)
• last
• size
• ids_reader
• empty?
• exists?

This can cause problems for certain usage patterns if we're no longer specifying eager loads:

> blogs = Blogs.limit(5).to_a
# SELECT * FROM blogs LIMIT 5

> blogs.each do |blog|
    if blog.posts.exists?
      puts blog.posts
    else
      puts 'No posts'
  end
# SELECT 1 AS one FROM posts WHERE blog_id = 1 LIMIT 1
# SELECT * FROM posts WHERE blog_id IN (1,2,3,4,5)

Notice the first call to blog.posts.exists? was executed via SQL because the posts association wasn't yet loaded. The fully_load option can be used to force ActiveRecord to fully load the association (and do any necessary automatic eager loading) when evaluating methods like exists?:

class Blog < ActiveRecord::Base
  has_many :posts, fully_load: true
end

Limitations

Goldiloader leverages the ActiveRecord eager loader so it shares some of the same limitations. See eager loading workarounds for some potential workarounds.

has_one associations that rely on a SQL limit

You should not try to auto eager load (or regular eager load) has_one associations that actually correspond to multiple records and rely on a SQL limit to only return one record. Consider the following example:

class Blog < ActiveRecord::Base
  has_many :posts
  has_one :most_recent_post, -> { order(published_at: desc) }, class_name: 'Post'
end

With standard Rails lazy loading the most_recent_post association is loaded with a query like this:

SELECT * FROM posts WHERE blog_id = 1 ORDER BY published_at DESC LIMIT 1

With auto eager loading (or regular eager loading) the most_recent_post association is loaded with a query like this:

SELECT * FROM posts WHERE blog_id IN (1,2,3,4,5) ORDER BY published_at DESC

Notice the SQL limit can no longer be used which results in fetching all posts for each blog. This can cause severe performance problems if there are a large number of posts.

Other Limitations

Associations with any of the following options cannot be eager loaded:

• limit
• offset
• finder_sql
• group (only applies to Rails < 5.0.7 and Rails 5.1.x < 5.1.5 due to a Rails bug)
• from (only applies to Rails < 5.0.7 and Rails 5.1.x < 5.1.5 due to a Rails bug)

Goldiloader detects associations with any of these options and disables automatic eager loading on them.

Eager Loading Limitation Workarounds

Most of the Rails limitations with eager loading can be worked around by pushing the problematic SQL into the database via database views. Consider the following example with associations that can't be eager loaded due to SQL limits:

class Blog < ActiveRecord::Base
  has_many :posts
  has_one :most_recent_post, -> { order(published_at: desc) }, class_name: 'Post'
  has_many :recent_posts, -> { order(published_at: desc).limit(5) }, class_name: 'Post'
end

This can be reworked to push the order/limit into a database view:

CREATE VIEW most_recent_post_references AS
SELECT blogs.id AS blog_id, p.id as post_id
FROM blogs, LATERAL (
  SELECT posts.id
  FROM posts
  WHERE posts.blog_id = blogs.id
  ORDER BY published_at DESC
  LIMIT 1
) p

CREATE VIEW recent_post_references AS
SELECT blogs.id AS blog_id, p.id as post_id, p.published_at AS post_published_at
FROM blogs, LATERAL (
  SELECT posts.id, posts.published_at
  FROM posts
  WHERE posts.blog_id = blogs.id
  ORDER BY published_at DESC
  LIMIT 5
) p

The models would now be:

class Blog < ActiveRecord::Base
  has_many :posts
  has_one :most_recent_post_reference
  has_one :most_recent_post, through: :most_recent_post_reference, source: :post
  has_many :recent_post_references, -> { order(post_published_at: desc) }
  has_many :recent_posts, through: :recent_post_reference, source: :post
end

class MostRecentPostReference < ActiveRecord::Base
  belongs_to :post
  belongs_to :blog
end

class RecentPostReference < ActiveRecord::Base
  belongs_to :post
  belongs_to :blog
end

Upgrading

From 0.x, 1.x

The auto_include association option has been removed in favor of the auto_include query scope method. Associations that specify this option must migrate to use the query scope method:

class Blog < ActiveRecord::Base
  # Old syntax
  has_many :posts, auto_include: false

  # New syntax
  has_many :posts, -> { auto_include(false) }
end

Status

This gem is tested with Rails 5.2, 6.0, 6.1 and Edge using MRI 2.6, 2.7 and 3.0.

Let us know if you find any issues or have any other feedback.

Change log

See the change log.

Contributing

1. Fork it
2. Create your feature branch (git checkout -b my-new-feature)
3. Commit your changes (git commit -am 'Add some feature')
4. Push to the branch (git push origin my-new-feature)
5. Create new Pull Request