class Post
  include Mongoid::Document
  field :title, :type => String
  has_many :comments do
    def update(selector, params)
      @target.each { |t| t.update_attributes(params) if t.matches?(selector) }
    end
  end
end

class Comment
  include Mongoid::Document
  field :body, :type => String
  belongs_to :post, :inverse_of => :comments
end

@post = Post.create(:title => 'Friday post')
@post.comments.create(:body => 'yay its friday')
@post.comments.create(:body => 'woohoo for tuesday!')

comments = @post.comments.where(:body => 'woohoo for tuesday')

comments.each { |comment| comment.body = "New Value" }
comments.first.body = "New Value"

@post.comments.update({:_id => params[:id]}, params[:comment])