如何在Rails 4中加快Dynamic Select

Question

I was able to create a dynamic dropdown list for Province and City by using an Adjacent list model that references to itself and jquery but it takes forever to load.

It works how I wanted it to work however; this process takes 7473.4ms to load every time it renders the form since I have 1718 rows in my locations table and it tries to load all 1718 of them one at a time.

I found out that what's making it load longer is the grouped_collection_select but I can't think of a faster alternative for this. Is there a query tweak you can advise me to make this form load faster?

The model looks like this:

class Location < ActiveRecord::Base
  has_many :books
  belongs_to :parent_location, class_name: Location
  has_many :child_locations, class_name: Location, foreign_key: "parent_id"
end

and the db (locations table) looks something like this but it has 1718 rows:

id  location         parent_id
1   Philippines 
2   Metro Manila        1
3   Abra                1
4   Caloocan City       2
5   City of Las Pinas   2
6   Pilar               3
7   Sallapadan          3

I was able to make the dynamic dropdown by using a "collection_select" for the provinces dropdown and a "grouped_collection_select" for the cities in my _form.html.haml:

= f.collection_select :location_id, Location.where(parent_id: "1"), :id, :name, {prompt: "Select a Province"}, id: "province"
= f.grouped_collection_select :location_id, Location.all, :child_locations, :name, :id, :name, {prompt: "Select a City/Municipality"}, id: "city"

then I used jQuery to show only the cities under the selected province in my js.cofee file:

jQuery ->
    city = $('#city').html()
    $('#province').change->
        province = $('#province :selected').text()
        options = $(city).filter("optgroup[label='#{province}']").html()
        if options
            $('#city').html(options)
        else
            $('#city').empty()

Answer 1

I was able to lower down the speed by using preload in grouped_collection :

it preloads all child_locations which made the loading time faster

 = f.grouped_collection_select :location_id, Location.preload(:child_locations), :child_locations, :name, :id, :name, {prompt: "Select a City/Municipality"}, id: "city"