ActiveStorage with ordered attachments using acts_as_list

application.rb

module MyApp
  class Application < Rails::Application
    config.to_prepare do
      ActiveStorage::Attachment.include OrderedAttachments
    end
  end
end

attachment_position.rb

class ActiveStorage::AttachmentPosition < ApplicationRecord
  belongs_to :attachment, class_name: "ActiveStorage::Attachment", foreign_key: :active_storage_attachment_id
  belongs_to :record, polymorphic: true

  acts_as_list scope: :record
end

has_ordered_attachments_concern.rb

module HasOrderedAttachments
  extend ActiveSupport::Concern

  class_methods do
    def has_ordered_attachments
      @has_ordered_attachments
    end

    def has_ordered_attachments_on?(name)
      has_ordered_attachments.include?(name)
    end

    def has_many_ordered_attachments(name)
      has_many_attached name 
      
      attachment_scope = proc { left_outer_joins(:active_storage_attachment_position).order(Arel.sql("COALESCE(active_storage_attachment_positions.position, 0)")) }
      blob_scope = proc { left_outer_joins(attachments: :active_storage_attachment_position).order(Arel.sql("COALESCE(active_storage_attachment_positions.position, 0)")) }
      self._reflections["#{name}_attachments"].instance_variable_set(:@scope, attachment_scope)
      self._reflections["#{name}_blobs"].instance_variable_set(:@scope, blob_scope)

      @has_ordered_attachments ||= Set.new
      @has_ordered_attachments << name

      generated_association_methods.class_eval <<-CODE, __FILE__, __LINE__ + 1
          # frozen_string_literal: true
          def #{name}
            @active_storage_attached ||= {}
            @active_storage_attached[:#{name}] ||= ActiveStorage::Attached::OrderedMany.new("#{name}", self)
          end
      CODE
    end
  end
end

migration.rb

# rails g model ActiveStorage::AttachmentPosition 
class CreateActiveStorageAttachmentPositions < ActiveRecord::Migration[7.0]
  def change
    create_table :active_storage_attachment_positions do |t|
      t.references :active_storage_attachment, null: false, foreign_key: true, index: { name: :index_attachment_positions_on_active_storage_attachment_id }
      t.references :record, null: false, polymorphic: true
      t.integer :position, null: false
      t.timestamps
    end
  end
end

ordered_attachments_concern.rb

module OrderedAttachments
  extend ActiveSupport::Concern

  included do
    has_one :active_storage_attachment_position, class_name: "ActiveStorage::AttachmentPosition", foreign_key: :active_storage_attachment_id, dependent: :delete

    after_create_commit do
      if ordered?(record_type, name)
        ActiveStorage::AttachmentPosition.create!(record_type: record_type, record_id: record_id, attachment: self)
      end
    end

    def ordered?(type, name)
      klass = type.constantize
      klass.respond_to?(:has_ordered_attachments) && klass.has_ordered_attachments_on?(name.to_sym)
    end
  end

  delegate :insert_at, :position=, :position, :move_lower, :move_higher, :move_to_bottom, :move_to_top,
           :remove_from_list, :increment_position, :decrement_position, :set_list_position, :first?, :last?, :in_list?,
           :not_in_list?, :default_position?, :higher_item, :lower_item, :lower_items, to: :active_storage_attachment_position
end

post.rb

class Post < ApplicationRecord
  include HasOrderedAttachments

  has_many_attached :images 
end