class FireHydrant
  include Mongoid::Document
  field :location, type: String
  has_and_belongs_to_many :cats, primary_key: :name
end

class Cat
  include Mongoid::Document
  field :name
  belongs_to :person, primary_key: :username
end

> fire_hydrant = FireHydrant.create(location: '221B Baker Street')
#<FireHydrant _id: 6174ccb84aec8bea5a39ba2a, location: "221B Baker Street", dog_ids: nil, cat_ids: nil>
> fire_hydrant.cats.build name: 'Meow'
#<Cat _id: 6174ce0a4aec8bea5a39ba2d, name: "Meow", person_id: nil>
> fire_hydrant.cat_ids
[BSON::ObjectId('6174ce0a4aec8bea5a39ba2d')] # instead of ['Meow']
> cats = [Cat.create(name: 'Kitty'), Cat.create(name: 'Kitten')]
[#<Cat _id: 6174ccbc4aec8bea5a39ba2b, name: "Kitty", person_id: nil>, #<Cat _id: 6174ccbc4aec8bea5a39ba2c, name: "Kitten", person_id: nil>]
> fire_hydrant.cats = cats
[#<Cat _id: 6174ccbc4aec8bea5a39ba2b, name: "Kitty", person_id: nil>, #<Cat _id: 6174ccbc4aec8bea5a39ba2c, name: "Kitten", person_id: nil>]
> fire_hydrant.cat_ids
[BSON::ObjectId('6174ccbc4aec8bea5a39ba2b'), BSON::ObjectId('6174ccbc4aec8bea5a39ba2c')] # instead of ['Kitty', 'Kitten']