Move embedded many to has_many mongoid.

embedded_many.rb

child_coll = Mongoid.default_session[:children]

Parent.all.each do |p|
  p.childs.all.each do |c|
    c.attributes['parent_id'] = p.id

    child_coll.insert c.attributes # save children to separate collection
  end

  p.childs = nil # remove embedded data
  p.save
end