class Artist
  include Mongoid::Document
  field :name, type: String
  embeds_many :instruments
end

class Instrument
  include Mongoid::Document
  field :name, type: String
  embedded_in :artist
end

# Find the first artist named "Syd Vicious" and create an embedded bass
# document for him. Of course he'll smash it within the next few minutes,
# but no worries we can delete it later.
Artist.where(name: "Syd Vicious").first.tap do |artist|
  artist.instruments.create(name: "Bass")
end