#public_method

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

What is the advantage of using Post.public_method(:new) rather than
just Post.new?

The first one actually returns the method itself as an object, to be called later.  
The second is calling #new and returns a new Post object.
Thus this:
  Post.new
is equivalent to this:
  Post.public_method(:new).call


Another Example - 

class Blog
  def title
    "Ruby DI"
  end
end

foo = Blog.new
foo.title
>> "Ruby DI"

bar = foo.public_method(:title)
bar.call
>> "Ruby DI"