https://gist.githubusercontent.com/palkan/03eb5306a1a3e8addbe8df97a298a466/raw/755f47f55084008a07252d7d670d712560fb89dd/association_loader.rb

# frozen_string_literal: true

module Common
  module GraphQL
    # See https://github.com/Shopify/graphql-batch/blob/master/examples/association_loader.rb
    #
    # Additionally added support for scopes.
    class AssociationLoader < ::GraphQL::Batch::Loader
      def self.validate(model, association_name)
        new(model, association_name)
        nil
      end

      def initialize(model, association_schema, scope_model: nil, scopes: nil)
        @model = model
        @association_schema = association_schema
        @association_name = extract_association_id(association_schema)

        if scope_model && scopes
          @preload_scope = scopes.reduce(scope_model) { |acc, scope| acc.public_send(scope) }
        end

        validate
      end

      def load(record)
        raise TypeError, "#{@model} loader can't load association for #{record.class}" unless record.is_a?(@model)
        return Promise.resolve(read_association(record)) if association_loaded?(record)
        super
      end

      # We want to load the associations on all records, even if they have the same id
      def cache_key(record)
        record.object_id
      end

      def perform(records)
        # Pass unique records to preloader to avoid duplicates
        preload_association(records.uniq)
        records.each { |record| fulfill(record, read_association(record)) }
      end

      private

      def validate
        unless @model.reflect_on_association(@association_name)
          raise ArgumentError, "No association #{@association_name} on #{@model}"
        end
      end

      def preload_association(records)
        ::ActiveRecord::Associations::Preloader.new.preload(
          records,
          @association_schema,
          @preload_scope
        ).then(&:first).then do |preloader|
          next unless @preload_scope

          # this commit changes the way preloader works with scopes
          # https://github.com/rails/rails/commit/2847653869ffc1ff5139c46e520c72e26618c199#diff-3bba5f66eb1ed62bd5700872fcd6c632
          preloader.send(:owners).each do |owner|
            preloader.send(:associate_records_to_owner, owner, preloader.records_by_owner[owner] || [])
          end
        end
      end

      def read_association(record)
        record.public_send(@association_name)
      end

      def association_loaded?(record)
        record.association(@association_name).loaded?
      end

      def extract_association_id(id_or_hash)
        return id_or_hash unless id_or_hash.is_a?(Hash)

        if id_or_hash.keys.size != 1
          raise ArgumentError, "You can only preload exactly one association! You passed: #{id_or_hash}"
        end

        id_or_hash.keys.first
      end
    end
  end
end