association trhough checkboxes

I need some help to make the right solution in rails. I have this
model in rails:

Customer -- has many -- Reservations
Customer -- has many -- Animals
Reservatiion -- has many -- Residences -- has one -- Animal

A customer can make a reservation for the animals he/she is owner of.
I made a view where I show the customer with two fields for the
reservations and the animals which belongs to the customer. I use
checkboxes for the animals to (un)check them for the reservation.

I get everything the work, however when some of the validations fails
I loose some information of my model. I use ryanb´s solution for
multiple models in a view. But in my model I check if the residence
must be made (if the checkbox is checked). I do something like this

[Code]
def new_residence_attributes(=new_residence_attributes)
  loop atrributes
    residences.build(attributes) if attribute[:status] == :reserved
  end
end
[/code]

So only the residences I build come back in my controller, so I loose
some information when I come back in my controller (only residences
which are reserved). The question how do I preserve the original model
created in the controller with the values of the view.

Thanks for your help in advance.

Greets,
Erwin