Repositories Provider

application_controller.rb

def repositories(repositories_provider = RepositoriesProvider)
  @repositories ||= repositories_provider.new()
end

rails_helper.rb

config.before(:each, type: :controller) do |example|
  repositories = controller.send(:repositories, TestRepositoriesProvider)
  define_singleton_method(:repositories) do
    repositories
  end
end

class TestRepositoriesProvider
  include RSpec::Mocks::ExampleMethods
  attr_reader :users

  def initialize
    @users = instance_double(UsersRepository)
  end
end

repositories_provider.rb

class RepositoriesProvider
  attr_reader :users

  def initialize
    @users = UsersRepository.new
  end
end

users_controller.rb

class UsersController < ApplicationController
  def index
    @users = repositories.users.find_by_id(params[:id])
  end
end

users_controller_spec.rb

require 'rails_helper'

describe UsersController do
  let(:user) { double(:user) }
  
  describe 'GET #show' do
    before do
      allow(repositories.users).to receive(:find_by_id).and_return(user)
    end

    it 'assigns users' do
      get :show, id: '7'

      expect(repositories.users).to have_received(:find_by_id).with('7')
    end
  end
end