Auto Preload

A gem to parse and run preload/includes/eager_load on your model from a JSON::API include string.

Add the gem to your Gemfile

gem 'auto_preload'

and run the bundle install command.

JSON::API allows API consumers to pass a query parameter, called include, to manually select which model associations should be resolved and returned in the output JSON.

This means that in your controller, you may have a dilemma:

• If the consumer requests an association that is not preloaded, Rails will run N+1 queries, slowing down the response
• You can't know, beforehand, which association may be requested by the consumer, since it's parametric
• You can just preload every possible association, but you'll end up making a lot of extra (redundant) queries in most cases.

This gem tries to fix this by parsing the include parameter and transforming it to a preload, includes or eager_load call in the model.

This gem adds to ActiveRecord classes a couple of utility methods that will help to preload associations.

To start using it, simply pass a JSON::API include string to the auto_preload class method of a model, and it will resolve it.

Here's an example:

# Models declaration
class User < ApplicationRecord
  has_many :articles
  has_many :comments
end

class Comment < ApplicationRecord
  belongs_to :user
end

class Article < ApplicationRecord
  belongs_to :user
  has_many :comments
end

# Now calling auto_preload on User
User.auto_preload('*') # Equivalent to preload(:articles, :comments)
User.auto_preload('articles.*') # Equivalent to preload(articles: [:user, :comments])

The same works also with eager_load and includes:

User.auto_eager_load('*') # Equivalent to eager_load(:articles, :comments)
User.auto_includes('*') # Equivalent to includes(:articles, :comments)

You can also use the keyword **, however it may take you to a loop.

For instance in this case, it would raise an error:

User.auto_preload('**') # Raises "Too many iterations reached (101 of 100)"

Since User resolves :articles, but Article declares belongs_to :user.

To solve this you can whitelist the associations you want to preload:

class Article < ApplicationRecord
  self.auto_preloadable = [:comments]
  belongs_to :user
  has_many :comments
end

class Comment < ApplicationRecord
  self.auto_preloadable = []
  belongs_to :user
end

Now you can safely use auto_preload:

User.auto_preload('**') # Equivalent to preload(:comments, articles: :comments)

By default, the resolution of the expressions passed to auto_preload methods is resolved by the ActiveRecord Adapter.

An Adapter is simply a class that, given a model, returns the list of the associations that can be preloaded.

The ActiveRecord Adapter uses reflect_on_all_associations to get this list.

In many circumstances, you don't want this. For instance, if you use ActiveModelSerializers gem, you want to resolve only associations that are declared in the serializer.

To do so, just change the default adapter using an initializer, in config/initializers/auto_preload.rb:

AutoPreload.config.adapter = AutoPreload::Adapters::Serializer.new

Of course, you can also declare your custom Adapters, simply creating a class that implements the method resolve_preloadables(model, options = {}) and returns a list of associations.

The gem is available as open source under the terms of the MIT License.

auto_preload is maintained by mònade srl.

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