_detail_subs.haml

%h1
  hello
- detail_subs.each do |sub|
  .form-group
    = sub.title
    = binding.pry
    = select_tag(:details, options_for_select(sub.details.pluck('title', 'id'), entry.detail_ids), multiple: true, class: 'chosen-select form-control')

:javascript
  $(function() {
    $(".chosen-select").chosen();
  })

_form.haml

.form-group
  = f.label :detail_category_ids, 'Select Amenities'
  = f.select(:detail_category_ids, options_from_collection_for_select(DetailCategory.all, 'id', 'title', f.object.detail_categories), {}, class: 'chosen-select-detail form-control')
.form-group
  #detail_subs

:javascript
  $(".chosen-select-detail").chosen().change(function(e){
  var detail_category_id = e.target.value
  $.get(
    '/admin/entries/#{ @entry.slug }/' + detail_category_id + '/detail_subs',
    {},
    function(data){
    console.log(data)
    }, 'script')
 });

detail.rb

class Detail < ApplicationRecord
  belongs_to :detail_sub
  has_many :entry_details
  has_many :entries, through: :entry_details
end

detail_category.rb

class DetailCategory < ApplicationRecord
  has_many :entry_detail_categories
  has_many :entries, through: :entry_detail_categories
  has_many :detail_subs
end

detail_sub.rb

class DetailSub < ApplicationRecord
  belongs_to :detail_category
  has_many :details
end

detail_subs_controller.rb

class Admin::DetailSubsController < Admin::PagesController
  def index
    @entry = Entry.friendly.find(params[:entry_id])
    detail_category = DetailCategory.find params[:detail_category_id]
    @detail_subs = detail_category.detail_subs

    respond_to do |format|
        format.js { render layout: false }
    end
  end
end

entries_controller.rb

  def entry_params
    params.require(:entry).permit(:title, :subtitle, :of_type, :intro, :content,
                                  :promo, :author, :address, :city, :state, :zipcode,
                                  :lat, :long, :directions, :website, :email, :synonyms,
                                  :facebook, :instagram, :twitter, :pinterest, :details,
                                  :meta_description, :meta_title, :meta_keywords,
                                  :facility, :group, :internet, :room, :room_amenities,
                                  :discounts, :slug, :type_id, :lake_assn,
                                  :featured_image, :is_featured, images: [],
                                  phone_numbers_attributes: [:id, :label, :number, :_destroy],
                                  videos_attributes: [:id, :url, :_destroy],
                                  hours_attributes: [:id, :title, :label_1, :label_2, :_destroy],
                                  prices_attributes: [:id, :label, :price, :_destroy],
                                  entry_detail_ids: [], related_entry_ids: [],
                                  detail_category_ids: [],  topic_ids: [],
                                  documents_attributes: [:id, :position, :title],
                                 )
  end

entry.rb

class Entry < ApplicationRecord
  ...
  
  has_many :entry_details
  has_many :details, through: :entry_details
  has_many :entry_detail_categories
  has_many :detail_categories, through: :entry_detail_categories
  
  ...
end

entry_detail.rb

class EntryDetail < ApplicationRecord
  belongs_to :entry
  belongs_to :detail
end

entry_detail_category.rb

class EntryDetailCategory < ApplicationRecord
  belongs_to :entry
  belongs_to :detail_category
end

index.js.haml

:plain
  $('#detail_subs').html("#{j render(partial: 'admin/entries/detail_subs', locals: {detail_subs: @detail_subs, entry: @entry})}")

When I submit I check for

EntryDetailCategory(id: integer, entry_id: integer, detail_category_id: integer, created_at: datetime, updated_at: datetime)

[9] pry(#<#Class:0x00007ffc8848dbb8>)> EntryDetailCategory.all
CACHE EntryDetailCategory Load (0.0ms) SELECT "entry_detail_categories".* FROM "entry_detail_categories"
↳ app/views/admin/entries/_form.haml:76
=> []
[10] pry(#<#Class:0x00007ffc8848dbb8>)>