PluckMap::Presenter

The PluckMap presenter provides a DSL for creating performant presenters. It is useful when a Rails controller action does little more than fetch several records from the database and present them in some other data format (like JSON or CSV).

Let's take an example. Suppose you have an action like this:

  def index
    @messages = Message.created_by(current_user).after(3.weeks.ago)
    render json: @messages.map { |message|
      { id: message.id,
        postedAt: message.created_at,
        text: message.text } }
  end

This, of course, instantiates a Message for every result, though we aren't really using a lot of ActiveRecord's features (in this action, at least). We instantiate all those objects on line 3 and then throw them away immediately afterward.

We can skip that step by using pluck:

  def index
    @messages = Message.created_by(current_user).after(3.weeks.ago)
    render json: @messages.pluck(:id, :created_at, :text)
      .map { |id, created, text|
        { id: id,
          postedAt: created_at,
          text: text } }
  end

In many cases, this is significantly faster.

But, now, if we needed to present a new attribute (say, channel), we have to write it four times:

  def index
    @messages = Message.created_by(current_user).after(3.weeks.ago)
-   render json: @messages.pluck(:id, :created_at, :text)
+   render json: @messages.pluck(:id, :created_at, :text, :channel)
-     .map { |id, created, text|
+     .map { |id, created, text, channel|
        { id: id,
          postedAt: created_at,
-         text: text } }
+         text: text,
+         channel: channel } }
  end

When we're presenting large or complex objects, the list of attributes we send to pluck or arguments we declare in the block passed to map can get pretty awkward.

The PluckMap::Presenter DSL is just a shortcut for generating the above pluck-map pattern in a more succinct way. The example above looks like this:

  def index
    @messages = Message.created_by(current_user).after(3.weeks.ago)
    presenter = PluckMap::Presenter.new do |q|
      q.id
      q.postedAt select: :created_at
      q.text
      q.channel
    end
    render json: presenter.to_h
  end

This DSL also makes it easy to make fields optional:

  def index
    @messages = Message.created_by(current_user).after(3.weeks.ago)
    presenter = PluckMap::Presenter.new do |q|
      q.id
      q.postedAt select: :created_at
      q.text
-     q.channel
+     q.channel if params[:fields] =~ /channel/
    end
    render json: presenter.to_h
  end

Installation

Add this line to your application's Gemfile:

gem "pluck_map"

And then execute:

$ bundle

Or install it yourself as:

$ gem install pluck_map

Development

After checking out the repo, run bin/setup to install dependencies. Then, run rake false 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/boblail/pluck_map.