gistfile1.txt

# app/models/address.rb
class Address < ActiveRecord::Base
end


# app/models/event.rb
class Event < ActiveRecord::Base
  belongs_to :address
end


# app/models/contact.rb
class Contact < ActiveRecord::Base
  belongs_to :address
end


# app/views/events/edit.html.haml
- form_for :event do |f|
  %p= f.text_field :name

  - f.fields_for :address do |a|
    %p= a.text_field :street
    # ...


# app/controllers/events_controller.rb
def update
  @event = Event.find(params[:id])
  @event.update_attributes(params[:event])
  if @event.save
    render :text => 'You got it.'
  else
    render :text => 'You lost it.'
  end
end


# error and parameters
ActiveRecord::AssociationTypeMismatch in EventsController#update
Address(#31131890) expected, got HashWithIndifferentAccess(#9690560)

{"commit"=>"Update",
 "_method"=>"put",
 "authenticity_token"=>"deb51c9504aa6b2090238780498fea660fa4d079",
 "id"=>"1",
 "event"=>{"name"=>"SuperEvent",
 "address"=>{"city"=>"Raleigh",
 "postal_code"=>"27239",
 "country"=>"USA",
 "street"=>"123 Streety St.",
 "state"=>"NC"}}}