context_example.rb

require 'spec_helper'
require 'ostruct'

class FakePostRepo
  attr_reader :posts

  def initialize
    @posts = []
  end

  def save(post)
    posts << post.merge(id: posts.count + 1)
    posts.count
  end
end

module ContextImport
  def self.[](*dependencies)
    Module.new do
      attr_reader :context

      def initialize(context: {}, **opts)
        @context = context
        super
      end

      dependencies.each do |dependency|
        define_method(dependency) do
          context.fetch(dependency)
        end
      end
    end
  end
end

Container = Dry::Container.new
Container.register :post_repo, FakePostRepo.new
Import = Dry::AutoInject(Container)

class CreatePost
  include Dry::Transaction
  include ContextImport[:current_user]
  include Import[:post_repo]

  step :save_post

  def save_post(attrs)
    id = post_repo.save(attrs.merge(user_id: current_user.id))
    Right(id)
  end
end

RSpec.describe CreatePost do
  it 'saves post' do
    use_case = CreatePost.new(context: { current_user: double(:user, id: 3) })
    post_id = use_case.call(title: 'Some Post').value

    expect(post_id).to eq 1
    expect(Container[:post_repo].posts).to eq([id: 1, user_id: 3, title: 'Some Post'])
  end
end