models.rb

class PetOwner < Mongoid::Document
  field :title
  has_one :address
end

class Address < Mongoid::Document
  field :street
  belongs_to :addressable, :inverse_of => :address
end

spec.rb

context "when address is a has one" do

  before do
    @owner = PetOwner.create(:title => "AKC")
    @address = Address.new(:street => "Fido Street")
    @owner.address = @address
    @owner.save    
  end

  it "is a single object and not an array" do
    @from_db = PetOwner.find(@owner.id)
    @from_db.address.should == @address
  end

end