Rails - Mar 2014 - 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!

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