Question

I have HABTM models Client and Book. Client model has a bookshelf_color attribute to indicate whether a client has a full set of books, part of them or none of them. Once the books on a bookshelf change, a callback set_bookshelf_color is supposed to reflect the change.

The question is why do I have to prefix bookshelf_color assignment in a private callback below with "self." to make it to work (as it does not otherwise)?

class Client < ActiveRecord::Base

  has_and_belongs_to_many :books, autosave: true, uniq: true,
    after_add: :set_bookshelf_color, after_remove: :set_bookshelf_color

  attr_accessible :id, :book_ids, :bookshelf_color

  private

    def set_bookshelf_color(book)
      if Book.pluck(:abbr).map{|b| books.map(&:abbr).map(&:to_s).include?(b.to_s)}.all?
        self.bookshelf_color = "green"
      elsif Book.pluck(:abbr).map{|b| books.map(&:abbr).map(&:to_s).include?(b.to_s)}.any?
        self.bookshelf_color = "yellow"
      else
        self.bookshelf_color = "red"
      end
    end
  # /private
end

Answer 1

Remember, your model properties are just instance variables behind the scenes – all we’re doing when we interact with them is calling the getter and setter methods set up by attr_accessor. When we write instance.bookshelf_color = "red" we’re sending the method bookshelf_color=, with the argument "red", to the receiver, instance.

Cool. What happens when we write just bookshelf_color = "red"?

In Ruby, bareword assignment is used to define local variables. Putting name = "value" in a method definition will define name in the local scope, rather than calling the name= method on self, even if that method exists.

class Foo
  attr_accessor :bar

  def set_bar(val)
    bar = val
  end
end

f = Foo.new
f.set_bar "baz"
f.bar # still nil

If I had specified an explicit receiver, self.bar = val, Ruby would have known I wanted to send self the method bar=, which would have had the desired result.

There’s some good further reading on the joys of self, and why you should use it, over on Joe Yates’ Blog.