ActiveModelPresenter

Have you had a problem that it's impossible to understand what methods/attributes are called in your Views? With ActiveModelPresenter you will have a full control of data that is exposed to views. Presenters (Model-View-Presenter, Clean Architecture) is one of the most useful patterns to improve supportability of your rails app.

1. List of exposed fields is explicitly defined
2. Presenters (serializers) can be an additional isolation layer for a view/frontend/api-related logic
3. Makes you 1 step closer to exposing a JSON API or allows you to DRY your code if you already do.
4. Easier testing - it's possible to unit-test a Presenter and check all attributes instead of greping an html.
5. DRY - Same as Service Objects they allow you to keep domain-related logic in 1 place and have an easy access to it
6. All queries and calculations happen in a presenter so no more crazy rendering time.
7. Everything is calculated only once. So you don't need to write memoizations like @foo ||= 2**100. It means caching a view will not make much sense

1. Presenter acts like a plain static ruby object but can be converted to a hash or a json at any moment.
2. Makes it easy to pass options to a serializer
3. Based on time-tested active_model_serializers gem
4. Collections are compatible with paginators like will_paginate and kaminari gems.

1. fields filtering - serialize/present only a subset of attributes that are listed in a serializer
2. ability to override/extend methods from a model
3. Caching - just add cache: true in your serializer. It checkes an updated_at attribute to refresh

1. Add the gem name to your Gemfile

   gem 'active_model_presenter'

2. Define the

3. Call presenter in your action

   def show  
     user = User.find(params[:id])
     @user = UserPresenter.show(user)
   end  

   Collections are handled in a same way.

   @users = UserPresenter.list(users)

   Here presenter returns a ActiveModelPresenter::Collection object, which is compatible with paginators.

See more information about serializers here about serializers syntax.

Presenter:

 # /app/presenters/post_presenter.rb
 class PostPresenter < ActiveModelPresenter::Base     
   def show(post)
     present(post, [:id, :title, :content, :comments_count]) do |result, model|
       result.comments = present(model.comments, [:message, :author_name])
     end
   end
   
   def list(posts)
     present(posts, [:id, :title, :content, :comments_count])
   end
 end 

present supports a block where you can make extend your presented object.

Presenters are using regular ActiveModelSerializer classes, like these:

 # /app/serializers/post_serializer.rb
 class PostSerializer < ActiveModel::Serializer
   attributes :id, :title, :content, :comments_count
     
   has_many :comments
     
   def comments_count
     comments.count
   end
 end 
 
 # /app/serializers/comment_serializer.rb
 class CommentSerializer < ActiveModel::Serializer
   attributes :id, :message, :author_name
     
   def author_name
     object.user.name
   end
 end   

Controller:

 # /app/controllers/posts_controller.rb
 class PostsController < ApplicationController
   def show  
     post = Post.find(params[:id])
     @post = PostPresenter.show(post)
   end
 end

` Views for show/index are same as default ones, but they can access only defined attributes.

 # /app/views/posts/show.html.erb

<h1><%= @post.title %></h1>
<div><%= @post.content %></div>

<h2>Comments (<%= @post.comments_count %>): </h2>
<div>
    <% @post.comments.each do |comment| %>
      <div>
        <div><%= comment.author_name %></div>
        <div><%= comment.message %></div>
      </div>  
    <% end %>
</div>

Method present accepts 3 attributes 1. Model or an object that inherits ActiveModelSerializers::Model 2. Array of fields to present. You can set it to nil to present all fields but it's not recommended. 3. Hash of params for serialization and for serializer instance. Available: 1.1 instance_options: {foo: :bar}. It will be available in the serializer methods as instance_options

We encourage you to create issues and to contribute to ActiveModelPresenter! Please discuss your ideas with the authors first.

ActiveModelPresenter is released under the MIT License.