A small mixin to get presenter functionality in any class.

presenter_mixin.rb

require 'active_support/concern'
require 'active_support/core_ext/class/attribute'

module Presenter
  extend ActiveSupport::Concern

  included do
    # class_attribute provides inheritance of the values
    class_attribute :presenters
    self.presenters = {}
  end

  class_methods do
    def presents(klass)
      presenters[klass] = self
    end

    def presenter(object)
      presenters[object.class]
    end

    def present(object)
      presenter(object)&.new(object) or raise NameError, "Presenter for #{object.class} is not registered"
    end
  end

  attr_reader :object

  def initialize(object)
    @object = object
  end
end

class BasePresenter
  include Presenter
end

class Foo
end

class FooPresenter < BasePresenter
  presents Foo
end

RSpec.describe 'presenters' do
  it 'works' do
    object = Foo.new
    presenter = BasePresenter.presenter(object)
    expect(presenter).to eq(FooPresenter)
    result = BasePresenter.present(object)
    expect(result.class).to eq(FooPresenter)
    expect(result.object).to eq(object)
  end
end

class AnotherPresenter
  include Presenter
end

RSpec.describe 'presenter scoping' do
  it 'has support for scopes' do
    object = Foo.new
    presenter = AnotherPresenter.presenter(object)
    expect(presenter).to be_nil
    expect { AnotherPresenter.present(object) }.to raise_error(NameError, "Presenter for Foo is not registered")
  end
end

# Possible additional features:
#
# * use SimpleDelegator or Forwardable
# * support multiple levels of inheritance (class B < A, but use APresenter instead of BPresenter)
# * Support for global scopes and not only local scopes
# * Depending on taste: the API could instead be `presents Foo, scope: :models` or `presents Foo, scope: ModelPresenters`
#   instead and avoid the subclassing