Chained select in ActiveAdmin

active_admin.js

if ($('body').hasClass('admin_subscriptions')) {
    $.post('/admin/users/1/subscriptions/get_subjects', function(data) {
      $('body').data('subscription.subjects', data);
    });

    $("#subscription_subject_type").on("change", function () {
      var $select_type = $("#subscription_subject_type"),
        $select_id = $("#subscription_subject_id");
      $("option[value]", $select_id).remove();
      $.each($('body').data('subscription.subjects'), function() {
        if (this.type == $select_type.val()) {
          $select_id.append('<option value=' + this.id + '>' + this.name + '</option>');
        }
      });
    });
  }

subscription.rb

ActiveAdmin.register Subscription do
  belongs_to :user
  navigation_menu :subscription

  scope("Taxonomy") { |scope| scope.where(subject_type: 'Taxonomy') }
  scope("Author") { |scope| scope.where(subject_type: 'Author') }

  config.filters = false

  collection_action :get_subjects, method: :post do
    render json: subjects
  end

  form do |f|
    f.semantic_errors

    f.inputs do
      f.input :subject_type, label: 'Type', as: :select2, placeholder: "Choose type", collection: self.controller.subject_types, input_html: { style: 'width: 300px;' }
      f.input :subject_id, label: 'Subject', as: :select2, placeholder: "Choose subject", collection: [], input_html: { style: 'width: 300px;' }
    end

    f.actions
  end

  index do
    selectable_column
    column "ID", :id
    column "Type", :subject_type
    column "Name" do |resource|
      resource.subject.name
    end
    column :created_at
  end

  controller do
    def subjects
      arr = []
      arr = arr + Author.all.map do |subject|
        {
          type: 'Author',
          id: subject.id,
          name: subject.name
        }
      end
      arr = arr + MuoCategory.all.map do |subject|
        {
          type: 'Category',
          id: subject.id,
          name: subject.name
        }
      end
      arr = arr + MuoTag.all.map do |subject|
        {
          type: 'Tag',
          id: subject.id,
          name: subject.name
        }
      end
      arr
    end

    def subject_types
      ['Author', 'Category', 'Tag']
    end
  end
end