[web2py] Cascading drop down list with a SQLFORM

[José Manuel López Muñoz]

Hi, 
I have a table with Offers that belongs to hotels that belongs to Cities . 
Now I have a SQLFORM that help me to create new offers and I want to make a 
cascading drop down list for all the hotels in a City when a City is 
created. 
I have found the way that Massimo recommends:

jQuery(function(){
  jQuery('#id_first_dropdown').change(function(){
     var selection = jQuery('#id_first_dropdown').val();
      jQuery.post('{{=URL('callbak')}}','value='+escapeURIComponent(
selection)).success(function(data){
          jQuery('#id_second_dropdown').html(data);
     });
  });
});


Although I barely understand the code (I'm very new with this), I don't 
know how to reference to a list drop down that is returned in the SQLFORM. 
To clarify the question this is my database model:
db.define_table('Offer',
SQLField('created_on', 'datetime', label="Día de creación de la oferta", 
writable=False, default=request.now),
                SQLField('valid_from_date', 'datetime', label="Oferta desde 
el día", default = request.now), 
                SQLField('valid_to_date', 'datetime', label="Oferta hasta 
el día", default = request.now),
                SQLField('selected','boolean', default=False, 
writable=False),  
                SQLField('name', writable=False, default=request.now),
                SQLField('city',db.City, label="Ciudad"), 
                SQLField('hotel',db.Hotel),
                format="%(id)s")

And I what to make a createOffer form with a drop down list of Cities that 
populate a Hotels drop down with the hotels that belongs to that City. 
I'm returning this:
def createOffer():
    hotelesUsuario=db(db.Hotel.managerID == auth.user_id)
    db.Offer.hotel.requires=IS_IN_DB(hotelesUsuario,'Hotel.id','%(name)s')
    form = SQLFORM(db.Offer)
    return dict(form=form, db = db) 



Thank you very much for your help 

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