ModelPresenter

ModelPresenter provides basic framework in creating a presenter. The goal of the gem is that the presenter can be used in any Ruby projects that feel it needs to use this pattern, not limited to Rails projects.

The presenter puts more emphasis on JSON representation of a business model object. But it can be used in classic HTML view just as easy.

The gem is not trying to provide a automagic way to look cool. Rather, it focuses on explicitly expressing the intent of a Presenter class. Please read the usage for the details

A presenter is a bridge between a business model and how it will be presented (as json, as HTML, as text, as PDF, etc.). A presenter covers logic that is necessary for presenting a business model, but such logic should not be put into the business model because it doesn't belong to.

For example, what attributes are going to be included in the JSON return is not a business model's concern. Another example, If the front end needs to have a person's gender in a humanly readable way (such as "Male" and "Female"), it is not the business model's concern. The business model knows it's gender in some way, it might choose to represent it as a boolean model.male? and model.female?. It is the responsibility of a presenter to provide what the front end needs. The Presenter may return the string Male or Female in its gender method.

A Presenter is not a place for generating HTML tags and code segments. An HTML file is where you write HTML code in.

Add this line to your application's Gemfile:

gem 'model_presenter'

And then execute:

$ bundle

An example presenter that uses the ModelPresenter::Base

class User
  include ModelPresenter::Base
  forward_from_model :last_name, :first_name, :email
  json_properties :name, :gender, :email

  def name
    "#{first_name} #{last_name}"
  end

  def gender
    model.gender == "M" ? "Male" : "Female"
  end
end

The forward_from_model defines methods attr1 and attr2 which calls the model.attr1 model.attr2 respectively.

The json_properties defines methods as_json which returns a hash in which the keys are the properties being passed in the json_properties and the value of a given property the presenter_object.property

An example output of as_json returns:

{
  name: "John Smith",
  gender: "Male",
  email: "jsmith@example.com"
}

Then one can use presenter_object.to_json to serilize it into a JSON string.

A presenter instance can be initialized with

user = User.new(user_model)
user.to_json

It always takes a model object as the only argument in the initializer. The model object is referred from within the presenter as presenter.model. It is a private attribute reader.

The presenter comes with a small helper for format the money. The usage:

class User
  include ModelPresenter::Base
  forward_from_model :amount_remains
  moneyize :amount_remains
end

What moneyize does it to define a new method formatted_amount_remains and return a formatted dollar display string.

NOTE: the amount_remains is supposed to be the amount in cents

So assuming amount_remains is 46780. The formatted_amount_remains returns $ 467.80

The presenter can define a has_many relationship

class User
  include ModelPresenter::Base
  has_many :groups, presenter_class: Presenters::Group
end

The macro will generates a groups methods, which will return an array. Each element of the array is an instance of Presenters::Group whose model is one of the group models that the user has.

In a Rails controller, I always initialize one and only one instance variable for using in the view - an instance of a Presenter class. The presenter has all necessary logic to make the view as dumb as possible. And all the logic can be unit tested just like any PORO, make your testing effort easy and enjoyable.

The gem provides some rspec macros for speeding up your test effort for your presenters. To use it, in your spec_helper.rb among your other setup:

require 'model_presenter/spec_support'
RSpec.configure do |config|
  ModelPresenter::SpecSupport.new(config).register
end

And in a presenter test, you can do:

describe User do
  forward_from_model_attributes :first_name, :last_name, :email
  as_json_attributes :first_name, :gender
  it_moneyize :amount_remains, :some_other_money_field
end

The forward_from_model_attributes macro will generate the following tests:

User
  #first_name
    returns the model.first_name
  #last_name
    returns the model.last_name
  #email
    returns the model.email

as_json_attributes macro will generate the following tests:

  #as_json
    has the key first_name with value set to presenter.first_name
    has the key gender with value set to presenter.gender

it_moneyize macro will generate the following tests:

  #formatted_amount_remains
    returns the formatted money in dollar
  #formatted_some_other_money_field
    returns the formatted money in dollar

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