Example

basic-polymorphism-example.rb

class Person
  include MongoMapper::Document
  one :address, :as => :addressable
end
class Address
  include MongoMapper::Document
  belongs_to :addressable, :polymorphic => true
end

# Console:
irb(main):003:0> p = Person.create
=> #<Person _id: BSON::ObjectId('4e4cbcd7d70fd90e24000005')>
irb(main):004:0> a = Address.create
=> #<Address _id: BSON::ObjectId('4e4cbcdcd70fd90e24000007')>
irb(main):005:0> a.addressable = p
=> #<Person _id: BSON::ObjectId('4e4cbcd7d70fd90e24000005')>
irb(main):011:0> a.save
=> true
irb(main):006:0> p.address
=> nil

# If I reverse the assigning like so, then things kind of work, but still not properly:
irb(main):013:0> p = Person.create
=> #<Person _id: BSON::ObjectId('4e4cbd7ad70fd90e2400000f')>
irb(main):014:0> a = Address.create
=> #<Address _id: BSON::ObjectId('4e4cbd7dd70fd90e24000011'), addressable_id: nil, addressable_type: nil>

irb(main):015:0> p.address = a
=> #<Address _id: BSON::ObjectId('4e4cbd7dd70fd90e24000011'), addressable_id: nil, addressable_type: nil, person_id: BSON::ObjectId('4e4cbd7ad70fd90e2400000f')>

irb(main):016:0> p.save
=> true
irb(main):017:0> a.save
=> true

irb(main):018:0> a
=> #<Address _id: BSON::ObjectId('4e4cbd7dd70fd90e24000011'), addressable_id: nil, addressable_type: nil, person_id: BSON::ObjectId('4e4cbd7ad70fd90e2400000f')>
irb(main):019:0> p
=> #<Person _id: BSON::ObjectId('4e4cbd7ad70fd90e2400000f')>

# Notice how it is assigning a "person_id" key to the address object, and leaving the addressable_* keys nil.