Inconsistent behavior of collection.new vs collection.build

I'm not sure how exactly collection.new should behave, as it's not really documented (at least not here: http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassM..., but it's behavior is inconsistent:

class User < ActiveRecord::Base
  has_many :friendships
  has_many :friends, :through => :friendships
end

class Friendship < ActiveRecord::Base
  belongs_to :user
  belongs_to :friend, :class_name => "User"
end

user.friendships.new
user.friendships # => []
user.friendships.build
user.friendships # => [#<Friendship id: nil, user_id: 1, friend_id: nil>]

# but
user.friends # => []
user.friends.new
user.friends # => [#<User id: nil...>]

I'm not sure how it should behave, but it would be great if it always behaved like for non-through associations. It works like this in Rails 3.0.5 and master.