An embeds_one within 1-level deep should use a positional operator

 ruby

require 'spec_helper'

class Sandwich

  include Mongoid::Document

  embeds_many :slices

end

class Slice

  include Mongoid::Document

  field :position, default: 0

  include Mongoid::Document

  embedded_in :sandwiches

  embeds_one :measure

end

class Measure

  include Mongoid::Document

  field :thickness

  embedded_in :slice

end

describe "relationship" do

  it "uses a positional operator" do

    sandwich = Sandwich.create!

    slice = Slice.new

    sandwich.slices << slice

    Moped.logger = Logger.new(STDOUT)

    Moped.logger.level = Logger::DEBUG

    slice.measure = Measure.new

  end

end

  MOPED: 127.0.0.1:27017 UPDATE 

    database=mongoid_test 

    collection=sandwiches 

    selector={"_id"=>"510af81c3b55524e16000001", "slices._id"=>"510af81c3b55524e16000002"} 

    update={"$set"=>{"slices.0.measure"=>{"_id"=>"510af81c3b55524e16000003"}}} flags=[]