class Author
  include Mongoid::Document

  field :name, type: String
  has_one :post
  accepts_nested_attributes_for :post
end

class Comment
  include Mongoid::Document

  field :body, type: String
  belongs_to :post
end

class Post
  include Mongoid::Document

  field :title, type: String
  belongs_to :author
  has_many :comments
  accepts_nested_attributes_for :comments
end

require 'rails_helper'

RSpec.describe Comment, type: :model do
  describe "accepts_nested_attributes relationships" do
    let(:author) { Author.create }
    let(:one_level_params) { { post_attributes: { title: 'test' } } }
    let(:two_levels_params) { { post_attributes: { comments_attributes: [ { body: 'test' } ] } } }

    it "creates a 1st-depth child model" do
      author.update_attributes(one_level_params)
      author.post.persisted?.should eq(true)
    end

    it "creates a 1st-depth child model, and a 2nd-depth child model" do
      author.update_attributes(two_levels_params)
      author.post.comments.count.should eq(1)
    end

    context "the 1st-depth child model already exists" do
      it "creates a 2nd-depth child model" do
        author.create_post(title: 'test') # Assume the first nested child relationship exists
        author.update_attributes(two_levels_params)
        author.post.comments.count.should eq(1) # This fails
      end
    end
  end
end

update_attributes

author.update_attributes({post_attributes: { 
    comments_attributes: [ 
        { body: 'test' }
    ]
})