example.rb

class Post
  include Mongoid::Document
  field :title, :type => String
  has_many :comments
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!')

# PROBLEM: how do i update the body attribute on that second post I added?
# @post.comments.where(:body => 'woohoo for tuesday!')
# The above query returns a criteria object with the document (Post in this case) that matches, but how do i select and update the proper comment?

comment = @post.comments.select { |c| c.body == 'woohoo for tuesday!' }
comment.body = "jk, I f'ing hate tuesdays"
@post.save