Saving associated objects without whitelisting their _id field

Hello,
I’ve been struggling for a while with a problem. I have the following (slightly simplified) structure:
Services

fields:

name

has_many :service_city_taxations, :dependent => :destroy
has_many :cities, :through => :service_city_taxations
accepts_nested_attributes_for :service_city_taxations, :allow_destroy => true
ServiceCityTaxations

fields:

service_id

city_id

local_taxation_rate

attr_accessible :city_id, :city_sales_tax_code, :city_sales_tax_rate, :service_id
belongs_to :city
belongs_to :service
Cities

fields:

name

has_many :service_city_taxations, :dependent => :destroy
has_many :services, :through => :service_city_taxations
Now, I work with services in the services view and I use nested forms to create ServiceCityTaxations directly along with my Services. I create and save them and everything works perfectly well.
However, I want to make this work without having to allow the city_id attribute in my ServiceCityTaxations’ attr_accessible, since that’s something the user picks himself. If I take city_id out of my attr_accessible parameters, Rails obviously ignores it during my mass assignment (update_attributes or save), since it’s an attribute that isn’t in my whitelist.
I was wondering if there’s some way for me to load up my City object in the controller before calling my update_attributes and then save my ServiceCityTaxations with the whole City object instead of only with the id or something like that. That way I could avoid whitelisting city_id at all, but there’s also the problem of dealing with nested form results from has_many associations in the controller, since the params hash gets quite messy.

I hope I was able to explain my situation. Any help is greatly appreciated.

Thanks!