Improve ActiveStorageLoader example for STI models

examples/active_storage_loader_sti.rb

@@ -0,0 +1,90 @@
+####
+# This is a loader for Active Storage attachments,
+# a little bit more complex than the `active_storage_loader.rb` example
+# because it supports STI (Single Table Inheritance) models where the
+# `has_one_attached` or `has_many_attached` is defined on any of the ancestor classes of the model.
+
+####
+# The model with an attached image and many attached pictures:
+####
+
+# class Event < ApplicationRecord
+#   has_one_attached :image
+#   has_many_attached :pictures
+# end
+
+####
+# An example data type using the AttachmentLoader:
+####
+
+# class Types::EventType < Types::BaseObject
+#   graphql_name 'Event'
+#
+#   field :id, ID, null: false
+#   field :image, String, null: true
+#   field :pictures, String, null: true
+#
+#   def image
+#     AttachmentLoader.for(:Event, :image).load(object.id).then do |image|
+#       Rails.application.routes.url_helpers.url_for(
+#         image.variant({ quality: 75 })
+#       )
+#     end
+#   end
+#
+#   def pictures
+#     AttachmentLoader.for(:Event, :pictures, association_type: :has_many_attached).load(object.id).then do |pictures|
+#       pictures.map do |picture|
+#         Rails.application.routes.url_helpers.url_for(
+#           picture.variant({ quality: 75 })
+#         )
+#       end
+#     end
+#   end
+# end
+
+class ActiveStorageLoader < GraphQL::Batch::Loader
+  attr_reader :record_type, :attachment_name, :association_type
+
+  def initialize(record_type, attachment_name, association_type: :has_one_attached)
+    super()
+    @record_type = record_type
+    @attachment_name = attachment_name
+    @association_type = association_type
+  end
+
+  def perform(record_ids)
+    attachments = ActiveStorage::Attachment.includes(:blob, :record).where(
+      record_type: ancestors_record_types, record_id: record_ids, name: attachment_name
+    )
+
+    if @association_type == :has_one_attached
+      attachments.each do |attachment|
+        fulfill(attachment.record_id, attachment)
+      end
+
+      record_ids.each { |id| fulfill(id, nil) unless fulfilled?(id) }
+    else
+      record_ids.each do |record_id|
+        fulfill(record_id, attachments.select { |attachment| attachment.record_id == record_id })
+      end
+    end
+  end
+
+  private
+
+  def ancestors_record_types
+    # Get all ancestor classes of record_type that are descendants of ActiveRecord::Base
+    # This is necessary because in a Single Table Inheritance (STI) setup,
+    # the `has_one_attached` or `has_many_attached`
+    # could be defined on any of the ancestor classes of the model, not just the model itself,
+    # which determines whether the `record_type` string is stored as the model's class name
+    # or the ancestor's class name.
+    # So we for any of the ancestor classes to ensure we don't miss the attachment
+    # we are looking for:
+
+    @record_type.to_s.constantize.ancestors.select { |ancestor|
+      ancestor < ActiveRecord::Base
+    }.map(&:to_s)
+  end
+end