Render new action with params - ruby-on-rails

Question

I have ajax-based select in my form, my form:
= form_for #car, :html => {:id => "new-car-form"} do |f|
- if #car.errors.any?
- if #car.errors.any?
.flash-notice
%b= "Ошибки в полях: #{#car.errors.count} ошибка(и)"
%ul
- #car.errors.each do |attr,msg|
%li= msg
.field
= f.label :vehicle_manufacturer_id, "Марка"
= f.select :vehicle_manufacturer_id, options_from_collection_for_select(VehicleManufacturer.all, :id, :name, #car.vehicle_manufacturer_id), {:prompt => "Выберите марку"}, required: true, id: "manufacturer-select"
.field
= f.label :vehicle_model_id, "Модель"
#models-area
= render :partial => 'models', :object => #models
and partial:
- if #models.present?
= select_tag "by_model", options_from_collection_for_select(#models, "id", "name", selected: #selected), :prompt => "Выберите модель", required: true, id: "model-select"
- else
= select_tag "by_model", nil, :prompt => "Выберите модель", required: true, id: "model-select", disabled: true
I test, and could have such case: when I didn't enter something and submit form - I see validation errors and my select values are nil, but how and in which action to append this something like this:
#models = VehicleModel.where(manufacturer_id: #car.vehicle_manufacturer_id)
#selected = #car.vehicle_model_id
I use rails4.
What I do wrong and how to solve my problem?

Answer 1

Try passing your form variable through to the partial; as the select_tag on its own won't be able to associate it's value with the form_for #car.
= render :partial => 'models', :object => #models, :form => f
Then, in the partial:
= form.select "by_model ..."
To be more specific, that form_for #car clause at the top of your form says 'wrap eveything in this block in a 'car' object when it is submitted'. So right now the form you're submitting has values like this:
car[vehicle_manufacturer_id]
car[vehicle_model_id]
by_model
But you need to wrap that last by_model attribute in the car array like the others:
car[vehicle_manufacturer_id]
car[vehicle_model_id]
car[by_model]