gistfile1.rb

class Article < ActiveRecord::Base
  has_many :authorships
  has_many :authors, through: :authorships

  def total_kudos
    authors.sum(&:kudos)
  end
end

# OK
author1 = Author.new kudos: 1
author2 = Author.new kudos: 2
a = Article.new
a.authors << author1
a.authors << author2
a.total_kudos.should == 3

# FAILS
author1 = Author.new kudos: 1
author2 = Author.new kudos: 2
a = Article.new
a.authorships.build author: author1
a.authorships.build author: author2
a.total_kudos.should == 3

# AR does not populate the #authors association when authorships are built