class Page
  include Mongoid::Document

  embeds_one :banner

  accepts_nested_attributes_for :banner, allow_destroy: true 
end

class Banner
  include Mongoid::Document
  
  embedded_in :page
  embeds_many :slides

  accepts_nested_attributes_for :slides, allow_destroy: true 
end

class Slide
  include Mongoid::Document

  embedded_in :banner
  embeds_many :contents

  accepts_nested_attributes_for :contents, allow_destroy: true 
end

class Content
  include Mongoid::Document

  embedded_in :slide

  field :value, type: String
end

'page' => {
   '_id' => '1',
  'banner_attributes' => {
    '_id' => '123',
    'slides_attributes' => [
      {
        '_id' => '1',
        'contents_attributes' => [
          { '_id' => '1', 'value' => 'some persisted value' },
          { 'value' => 'new content here, there is no _id yet' }
        ]
      },
      {
        'contents_attributes' => [
          { 'value' => 'new content in a newly created slide' }
        ]
      }
    ]
   }
}

Moped::Errors::OperationFailure (The operation: #<Moped::Protocol::Command
   @length=87
   @request_id=594
   @response_to=0
   @op_code=2004
   @flags=[]  
   @full_collection_name="web_builder_development.$cmd"
   @skip=0  
   @limit=-1
   @selector={:getlasterror=>1, :w=>1}
   @fields=nil>

failed with error 16837: "Cannot update 'banner.slides.0.contents' and 'banner.slides' at the same time"