Explains how mongoid embeds_many is breaking when nesting more than 1 level deep

mongoid_nested_embeds_many.rb

class Author
  include Mongoid::Document
  field :name, :type => String
  embeds_many :posts
end

class Post
  include Mongoid::Document
  field :title, :type => String
  embedded_in :author
  embeds_many :comments
  accepts_nested_attributes_for :comments
end

class Comment
  include Mongoid::Document
  field :text, :type => String
  embedded_in :post
end


# EXAMPLE 1
# nested_attributes will not save embeds_many relations more than 1 level deep

?> author = Author.create(:name => "Bob")
=> #<Author _id: 4d3bdf2998cefdf42d000001, name: "Bob">

?> post1 = author.posts.build(:title => "My first blog post", :comments_attributes => { "1" => { "text" => "Nice post man" }, "2" => { "text" => "Spam spam spam" } })
=> #<Post _id: 4d3bdf3698cefdf42d000002, title: "My first blog post">

?> post1.comments
=> [#<Comment _id: 4d3bdf3698cefdf42d000003, text: "Nice post man">, #<Comment _id: 4d3bdf3698cefdf42d000004, text: "Spam spam spam">]

?> post1.save
=> true

?> author.reload
=> #<Author _id: 4d3bdf2998cefdf42d000001, name: "Bob">

?> author.posts.last
=> #<Post _id: 4d3bdf3698cefdf42d000002, title: "My first blog post">

?> author.posts.last.comments
=> []


# EXAMPLE 2
# Similarly, the following will fail even when not using nested_attributes.

?> post2 = author.posts.build(:title => "Another fine post")
=> #<Post _id: 4d3be00d98cefdf42d000005, title: "Another fine post">

?> post2.comments << [ Comment.new(:text => "Profound indeed"), Comment.new(:text => "More spam") ]
=> [#<Comment _id: 4d3be03098cefdf42d000006, text: "Profound indeed">, #<Comment _id: 4d3be03098cefdf42d000007, text: "More spam">]

?> post2.comments.size
=> 2

?> post2.save
=> true

?> author.reload
=> #<Author _id: 4d3bdf2998cefdf42d000001, name: "Bob">

?> author.posts.last
=> #<Post _id: 4d3be00d98cefdf42d000005, title: "Another fine post">

?> author.posts.last.comments
=> []


# Notice above that comments are being pushed onto the post before it gets persisted on author.
# This is similar to how the NestedAttributes::Many builder processes nested attributes from
# the bottom up, which often results in new objects being pushed into an un-persisted parent.

# A patch for the NestedAttributes::Many builder might look something like this.
# However, the options hash is getting lost on subsequent calls to initialize.
# Perhaps a better way would be to implement this functionality inside Array.push (i.e. <<)


module Mongoid::Relations::Builders::NestedAttributes::Many < NestedBuilder

  def build(parent)
    @existing = parent.send(metadata.name)
    if over_limit?(attributes)
      raise Errors::TooManyNestedAttributeRecords.new(existing, options[:limit])
    elsif metadata.embedded? && !parent.persisted?
      # If the parent is not yet persisted, call save and
      # then recursively set it's nested attributes
      parent.upsert
      parent.send("#{metadata.name}_attributes=", @raw_attributes)
    else
      attributes.each { |attrs| process(attrs[1]) }
    end
  end

  def initialize(metadata, attributes, options = {})
    @raw_attributes = attributes  # Probably a better way to do this
    @attributes = attributes.with_indifferent_access.sort do |a, b|
      a[0] <=> b[0]
    end
    @metadata = metadata
    @options = options
  end
  
  ...

end