# items controller
  def update
    params[:positions].uniq.each_with_index do |id, index|
      # Changed @column to @list
      @list.items.find(id).update(position: index + 1)
    end
    ...
  end
  ...
  def set_item
    # changed @column to @list
    @item = @list.items.find(params[:id])
  end

# models/list.rb
# add association to items from a list
has_many :items, through: :columns

# Terminal
rails g migration add_position_to_items position:integer
rails db:migrate

# db/migrate/XXXXXX_add_position_to_items.rb
class AddPositionToItems < ActiveRecord::Migration[6.0]
  def change
    add_column :items, :position, :integer, default: 0
  end
end

# javascript/controllers/draggable_controller.js
import { Controller } from 'stimulus'
import { Sortable } from '@shopify/draggable'

export default class extends Controller {
  static targets = ['column', 'item']
  initialize() {}
  connect() {
    if (this.hasItemTarget) {
      this.itemTargets.forEach(item => {
        item.setAttribute('style', 'z-index: 1000;')
      })
      const sortable = new Sortable(this.columnTargets, {
        draggable: 'li'
      })
      sortable.on('sortable:stop', function(event) {
        let array = Array.from(event.newContainer.children)
        array.forEach((item, index) => {
          if (item.classList.contains('draggable--original')) {
            array.splice(index, 1)
          } else if (item.classList.contains('draggable-mirror')) {
            array.splice(index, 1)
          }
        })
        let positions = array.map(item => item.dataset.id)
        let url = event.dragEvent.source.getAttribute('data-url')
        let column = event.newContainer.getAttribute('data-id')
        let data = { item: { column_id: column }, positions: positions }
        let token = document.head.querySelector('meta[name="csrf-token"]').getAttribute('content')
        fetch(url, {
          method: 'PUT',
          credentials: 'same-origin',
          headers: {
            "X-CSRF-Token": token,
            "Accept": "application/json",
            "Content-type": "application/json"
          },
          body: JSON.stringify(data)
        })
      })
    }
  }
  disconnect() {}
}

# views/items/_item.html.erb
<%= content_tag :li, data: { target: 'draggable.item',
                             id: item.id,
                             url: list_column_item_path(list, column, item) } do %>

# controllers/items_controller.rb
  def update
    params[:positions].uniq.each_with_index do |id, index|
      @list.items.find(id).update(position: index + 1)
    end
    if @item.update(item_params)
      respond_to do |format|
        format.html { redirect_to @list, notice: 'Item was successfully updated.' }
        format.json {}
      end
    else
      render :edit
    end
  end

# controllers/lists_controller.rb
    def set_list
      @list = current_company.lists
                             .includes(columns: :items)
                             .order('columns.id, items.position ASC')
                             .find(params[:id])
    end