Conditional Form in Web2Py

I need a controller and view to make a form that has conditional dropdowns. my table definitions: ##### Adverts_Type = db.define_table('adverts_type', Field('label', 'string'), Field('description', 'text'), ) States = db.define_table('states', Field('label', 'string'), ) Cities = db.define_table('cities', Field('state_id', [login to view URL]), Field('label', 'string'), ) Zones = db.define_table('zones', Field('city_id', [login to view URL]), Field('label', 'string'), ) Adverts = db.define_table('adverts', Field('owner_id', 'integer'), Field('type_id', adverts_type), Field('description', 'text'), Field('image', 'upload'), Field('city_id', 'integer'), Field('zone_id', 'integer'), ) [login to view URL] = IS_IN_DB(db(Adverts_Type), '[login to view URL]', '%(label)s') [login to view URL] = IS_IN_DB(db(States), '[login to view URL]', '%(label)s') [login to view URL] = IS_IN_DB(db(Cities), '[login to view URL]', '%(label)s') #### I want a controller named "insert_add" that create a adverts form with the functionality of set a state and automatically fill a dropdown with all cities from selected state, set a city and automatically fill a dropdown with all zones from that selected cities...