DataMapper: order results by a property on a ManyToMany relationship

file.rb

DataMapper::Logger.new($stdout, :default)
DataMapper.setup(:default, "sqlite::memory:")

class Author
  include DataMapper::Resource

  property :id,     Serial
  property :email,  String

  has n, :posts
  has n, :comments
  has n, :post_comments, :through => :posts, :via => :comments, :model => "Comment", :inverse => :post_author
end

class Post
  include DataMapper::Resource

  property :id,     Serial
  property :body,   String

  belongs_to :author
  has n, :comments
end

class Comment
  include DataMapper::Resource

  property :id,     Serial
  property :body,   String

  belongs_to :author
  belongs_to :post
  has 1, :post_author, :through => :post, :via => :author, :model => "Author", :inverse => :post_comments
end

DataMapper.auto_migrate!

# This works, but it's a bit inconvenient to be this explicit
# Author.all(:order => Author.comments.body.desc, :fields => [Author.id, Author.email], :links => [Author.relationships[:comments].inverse])

# This is as terse as I could get the ordering on a :through (ManyToMany) relationship
Author.all(:order => Author.post_comments.body.desc, :links => [Author.relationships[:comments].inverse])