Rails 4: How to update a collection_select based on another collection_select through AJAX?

Question

The problem: I need to filter a collection of Units based upon the selection of a collection of Organizations.

Upo

Answer 1

I made it, the solution was pretty simple, but the lack of updated material regarding this simple issue made it a chore bigger than it should have:

config/routes.rb

 get 'filter_units_by_organization' => 'projects#filter_units_by_organization'

controllers/projects_controller.rb

def filter_units_by_organization
  @filtered_units = Unit.where(organization_id: params[:selected_organization])
end

views/projects/filter_units_by_organization.js.erb

$('select#project_unit_id').html('<%= j options_from_collection_for_select(@filtered_units, :id, :name) %>');

assets/javascripts/application.js

$(function() {
    $("select#project_organization_id").on("change", function() {
        $.ajax({
            url:  "/filter_units_by_organization",
            type: "GET",
            data: { selected_organization: $("select#project_organization_id").val() }
        });
    });
});

views/projects/_form.html.erb

<div class="field">
    <%= f.label :name %><br>
    <%= f.text_field :name %>
  </div>
  <div class="field">
    <%= f.label :description %><br>
    <%= f.text_area :description %>
  </div>
  <div class="field">
    <%= f.label :organization_id %><br>
    <%= f.collection_select :organization_id, Organization.all, :id, :name, { prompt: 'Please select' } %>
  </div>
  <div class="field">
    <%= f.label :unit_id %><br>
    <%= f.collection_select :unit_id, Unit.all.where(organization_id: :organization_id), :id, :name %>
  </div>
  <div class="actions">
    <%= f.submit %>
  </div>

Answer 2

Instead of placing an duplicate organization_id on the projects table you should set the relationship up to go through the Unit model.

class Organization < ActiveRecord::Base
  has_many :units
  has_many :projects, through: :units
end

class Unit < ActiveRecord::Base
  belongs_to :organization
  has_many :projects
end

class Project < ActiveRecord::Base
  belongs_to :unit
  has_one :organizaton, through: :unit
end

This avoids the awkward issue where you have to ensure that a unit and a project have the same organizaton_id.

This also means that you no longer need a input to select the organization when creating a Project - just the unit.