many_proxy_custom_method_call.rb

require 'rubygems'
require 'mongo_mapper'

  MongoMapper.connection = Mongo::Connection.new('localhost',27017)
  MongoMapper.database = "test"

class User
    include MongoMapper::Document
    key :name, String
    many :ideas do
        def roots
            map do |i|
                i.idea_roots.inspect
            end
        end
    end
end

class Idea
    include MongoMapper::Document
    key :content, String
    key :user_id, ObjectId
    def idea_roots
        "#{self.content} roots... "
    end
    belongs_to :user
end

User.collection.remove
Idea.collection.remove

user = User.new(:name => "1")
user.ideas = [Idea.new(:content => "one"), Idea.new(:content=>"two"), Idea.new(:content => "three")]
user.save

puts User.first.ideas.roots