Datamapper DataMapper

Here's a sample use case where this would be invaluable -- being able to specify fields to load on a belongs_to relationship to avoid eager loading data we don't need.

    posts = Post.all

    posts.size # => 842
    posts.user.count # => 840

    posts.each do |post|
      puts "#{post.title}, by #{post.user.name}"
      # This will eager load all user data for all
      # 840 users:
      #   select * from users where user_id in (...)
    end

    # proposal:

    posts.each do |post|
      user = post.user(:fields => :name)
      puts "#{post.title}, by #{user.name}"
      # This will eager load only name (and id) for the users:
      #   select id, name from users where user_id in (...)
    end