Question

I’m reading through Avdi’s objects on rails book and don’t understand a section of sample code.

He creates a class like so I guess for dependency injection purposes:

class Blog
  # ...
  attr_writer :post_source
  # ...
  private
  def post_source
    @post_source ||= Post.public_method(:new)
  end
end

Then he writes the following spec

# spec/models/blog_spec.rb
require 'ostruct'
describe Blog do
  # ...
  describe "#new_post" do
    before do
      @new_post = OpenStruct.new
      @it.post_source = ->{ @new_post }
    end
    it "returns a new post" do
      @it.new_post.must_equal @new_post
    end
    it "sets the post's blog reference to itself" do
      @it.new_post.blog.must_equal(@it)
    end
  end
end

I don’t understand why he uses @it.post_source = ->{ @new_post }

Why didn’t he just use something like @it.post_source = OpenStruct.public_method(:new) which would be similar to his Blog class code which has @post_source ||= Post.public_method(:new)

Is there a reason for this?

Answer 1

->{ @new_post } is a lambda that returns the instance stored in @new_post.

Post.public_method(:new) would return the constructor method of Post

Passing in the lambda for the class to use lets you have control of the instance that is returned. Passing in a class’ constructor means you don’t know what instance it will get, just that it will be of the class you specified.