poly.rb

require 'rubygems'
require 'mongo_mapper'
MongoMapper.database = 'test-poly'

class Company
  include MongoMapper::Document

  many :addresses, :as => :addressable
end

# Only including to show that this is not just a contrived example
class Person
  include MongoMapper::Document

  many :addresses, :as => :addressable
end

class Address
  include MongoMapper::Document

  key :addressable_id,   ObjectId
  key :addressable_type, String

  belongs_to :addressable, :polymorphic => true
end

# In console:

a = Address.new
c = Company.new

a.addressable = c

c.addresses #=> [] <-- Should be a, but is not

# Seems to work after a saves
a.save
p c.addresses

# However, this works:

c.addresses << a
c.addresses #=> a

p c.addresses