ApiPresenter

A much longer readme is coming, including best practices and cautions, but in the meantime lets keep it simple...

When creating RESTful APIs for web or mobile clients, there are a couple of common use cases that have emerged:

• Allow inclusion of associated data to mitigate number of requests
• Include permissions so that the client can intelligently draw its UI (ex: edit/delete buttons), while maintaining a single source of truth

ApiPresenter does both of these things, plus a bit more.

Installation

Add this line to your application's Gemfile:

gem 'api_presenter'

And then execute:

$ bundle

Or install it yourself as:

$ gem install api_presenter

Usage

We'll use a simple blog as the usage example for this gem. The blog has the following model structure:

class Category < ActiveRecord::Base
  has_many :sub_categories
  has_many :posts, through: :sub_categories
end

class SubCategory < ActiveRecord::Base
  belongs_to :category
  has_many :posts
end

class Post < ActiveRecord::Base
  belongs_to :sub_category
  belongs_to :creator, class_name: 'User'
  belongs_to :publisher, class_name: 'User'
end

class User < ActiveRecord::Base
  has_many :created_posts, class_name: 'Post', foreign_key: 'creator_id'
  has_many :published_posts, class_name: 'Post', foreign_key: 'publisher_id'
end

When clients request posts (the primary collection), they may want any or all of the above data for those posts.

Create your Presenter

class PostPresenter < ApiPresenter::Base
  def associations_map
    {
      categories:     { associations: { sub_category: :category } },
      sub_categories: { associations: :sub_category },
      users:          { associations: [:creator, :publisher] }
    }
  end

  def policy_methods
    [:update, :destroy]
  end

  # def policy_associations
  #   :user_profile
  # end
end

Presenters can define up to three methods:

• associations_map The includable resources for the ActiveRecord model (Post, in this case). Consists of the model name as key and traversla required to preload/load them. In most cases, the value of associations will correspond directly to associations on the primary model.
• policy_methods A list of Pundit policy methods to resolve for the primary collection.
• policy_associations Additional records to preload in order to optimize policies that must traverse asscoiations.

Enable your controllers

ApiPresenter provides a controller concern that executes the Presenter. This process analyzes your params, preloads records as needed, and produces a @presenter object you can work with.

class ApplicationController
  include ApiPresenter::Concerns::Presentable
end

class PostsCOntroller < ApplicationController
  def index
    posts = PostQuery.records(current_user, params)
    present posts
  end

  def show
    post = Post.find(params[:id])
    present post
  end
end

Render the result

How you ultimately render the primary collection and the data produced by ApiPresenter is up to you. @presenter has the following properties:

• collection The primary collection that was passed into the presenter.
• total_count When using Kaminari or another pagination method that defines a total_count property, returns unpaginated count. If the primary collection is not an ActiveRecord::Relation, simply returns the number of records.
• included_collection_names Convenience method that returns an array of included collecton model names.
• included_collections A hash of included collections, consisting of the model name and corresponding records.
• policies An array of resolved policies for the primary collection.

Here's an example of how you might render this using JBduiler:

api/posts/index.json.jbuilder

json.posts(@presenter.collection) do |post|
  json.partial!(post)
end
json.partial!("api/shared/included_collections_and_meta", presenter: @presenter)

api/shared/included_collections_and_meta

presenter.included_collections.each do |collection_key, collection|
  json.set!(collection_key, collection) do |record|
    json.partial!(record)
  end
end

json.meta do
  json.total_count(presenter.total_count)
  json.policies presenter.policies
end

Advanced Usage

Conditional includes

There are a number of ways you can conditionally include resources, depending, for insatnce, on user type.

Add conditions inside associations_map method

class PostPresenter < ApiPresenter::Base
  def associations_map
    current_user.admin? ? admin_associations_map : user_associations_map
  end

  private

  def user_associations_map
    {
      sub_categories: { associations: :sub_category },
      users:          { associations: [:creator, :publisher] }
    }
  end

  def admin_associations_map
    {
      categories:     { associations: { sub_category: :category } },
      sub_categories: { associations: :sub_category },
      users:          { associations: [:creator, :publisher] }
    }
  end
end

Use condition property within association_map definition

Via inline string:

class PostPresenter < ApiPresenter::Base
  def associations_map
  {
    categories:     { associations: { sub_category: :category }, condition: 'current_user.admin?' },
    sub_categories: { associations: :sub_category },
    users:          { associations: [:creator, :publisher] }
  }
  end
end

Via method call:

class PostPresenter < ApiPresenter::Base
  def associations_map
  {
    categories:     { associations: { sub_category: :category }, condition: :admin? },
    sub_categories: { associations: :sub_category },
    users:          { associations: [:creator, :publisher] }
  }
  end

  private

  def admin?
    current_user.admin?
  end
end

Control it from your policy

class CategoryPolicy < ApplicationPolicy
  def index?
    user.admin?
  end
end

TODO

• More doc
• Decouple from Pundit
• Make index policy checking on includes optional
• Allow custom collection names
• Add test helper to assert presenter was called for a given controller action

Development

After checking out the repo, run bin/setup to install dependencies. Then, run rake rspec to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to rubygems.org.

Contributing

Bug reports and pull requests are welcome on GitHub at https://github.com/[USERNAME]/api_presenter.

License

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