Add ActiveRecord 6.0 support

graphql-preload.gemspec

@@ -21,7 +21,7 @@ Gem::Specification.new do |spec|
   spec.executables   = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }
   spec.require_paths = ['lib']
 
-  spec.add_runtime_dependency 'activerecord', '>= 4.1', '< 6'
+  spec.add_runtime_dependency 'activerecord', '>= 4.1', '< 7'
   spec.add_runtime_dependency 'graphql', '>= 1.8', '< 2'
   spec.add_runtime_dependency 'graphql-batch', '~> 0.3'
   spec.add_runtime_dependency 'promise.rb', '~> 0.7'

lib/graphql/preload/loader.rb

@@ -35,7 +35,23 @@ def perform(records)
       end
 
       private def preload_association(records)
-        ActiveRecord::Associations::Preloader.new.preload(records, association, preload_scope)
+        preloader = ActiveRecord::Associations::Preloader.new.preload(records, association, preload_scope).first
+        return unless preload_scope
+        return if Gem::Version.new(ActiveRecord::VERSION::STRING) < Gem::Version.new("6.0.0")
+
+        # See https://github.com/rails/rails/issues/36638 for details
+        # Solution adapted from https://gist.github.com/palkan/03eb5306a1a3e8addbe8df97a298a466
+        if preloader.is_a?(::ActiveRecord::Associations::Preloader::AlreadyLoaded)
+          raise ArgumentError,
+              "Preloading association twice is not possible. " \
+              "To resolve this add `preload #{association.inspect}` to the GraphQL field definition."
+        end
+
+        # 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
 
       private def preload_scope