Nested Models Error in Rails 3

I have two models App and Contact. App has a has_one relationship with Contact. I have declared the accepts_nested_attributes_for clause in the App model for Contact. Now, in the apps_controller, if I use the build method on the app object, I get an error for the the nil class, even though I had declared the relationship.

App.rb

class App < ActiveRecord::Base
  has_one :contact_person, :dependent => :destroy
  accepts_nested_attributes_for :contact_person
end

ContactPerson.rb

class ContactPerson < ActiveRecord::Base
  belongs开发者_如何学C_to :app
end

apps_controller.rb

def new
  @app = App.new
  @app.contact_person.build
end

Could you please point me out whether am I doing anything incorrectly. I have used nested models before but have not encountered this error.

I am supposed to use @app.build_contact_person instead of @app.contact_person.build. In this way, it worked :)

Declaring association does not automatically creates it:

class App < ActiveRecord::Base
  has_one :contact_person, :dependent => :destroy
  accepts_nested_attributes_for :contact_person
  # Adding this line should work
  after_create { self.contact_person = ContactPerson.new }
end