form_for without ActiveRecord, form action not updating

I'm using an API instead of a database, so I'm not using ActiveRecord but ActiveModel (I mostly did like here: railscasts.com/episodes/219-active-model)

Thing is, when I try to edit an item (in my case a parking), the action of the form still remains the action of the create and not update.

so when I go on /parkings/2/edit to edit a parking, the form is still:

<form accept-charset="UTF-8" action="/parkings" class="form-horizontal" id="new_parking" method="post">

when it should be more like that with the put hidden field and the parkings/2 as the action:

<form accept-charset="UTF-8" action="/parkings/2" class="form-horizontal" id="edit_parking" method="post"><div style="margin:0;padding:0;display:inline"><input name="utf8" type="hidden" value="&#x2713;" /><input name="_method" type="hidden" value="put" />

Anybody knows where the method & action of the form_for is set according to the route? What I'm trying to do is be as close as if I was using ActiveRecord with a database.

Here is some code :

_form.html.erb

<%= form_for(@parking, :html => { :class => "form-horizontal" }) do |f| %>
...
<% end %>

edit.html.erb & new.html.erb, simply has

<%= render 'form' %>

Controller

class ParkingsController < ApplicationController
  def index    
    @parkings = Parse.get("Parking")

    respond_to do |format|
      format.html
      format.json { render :json => @parking }
    end
  end

  def new
    @parking = Parking.new

    respond_to do |format|
      format.html
      format.json { render :json => @parking }
    end
  end

  def edit
    @parking = Parking.find(params[:id])

    respond_to do |format|
      format.html
      format.json { render :json => @parking }