theorygeek · June 6, 2025 19:25 · Apr 26, 2017 · Apr 26, 2017 · Apr 26, 2017
diff --git a/post_type.rb b/post_type.rb
@@ -2,10 +2,10 @@
   name "Post"
 
   field :comments, !types[!CommentType] do
-    preload :comments
+    preload comments: :author
     resolve -> (post, args, ctx) { post.comments }
   end
 
   # Or you can use the more terse syntax:
-  field :comments, !types[!CommentType], preload: :comments, property: :comments
+  field :comments, !types[!CommentType], preload: { comments: :author }, property: :comments
 end
diff --git a/post_type.rb b/post_type.rb
@@ -0,0 +1,11 @@
+PostType = GraphQL::ObjectType.define do
+  name "Post"
+
+  field :comments, !types[!CommentType] do
+    preload :comments
+    resolve -> (post, args, ctx) { post.comments }
+  end
+
+  # Or you can use the more terse syntax:
+  field :comments, !types[!CommentType], preload: :comments, property: :comments
+end
diff --git a/association_loader.rb b/association_loader.rb
@@ -0,0 +1,23 @@
+# frozen_string_literal: true
+class AssociationLoader < GraphQL::Batch::Loader
+  attr_reader :klass, :association
+
+  def initialize(klass, association)
+    raise ArgumentError, "association to load must be a symbol (got #{association.inspect})" unless association.is_a?(Symbol)
+    raise ArgumentError, "cannot load associations for class #{klass.name}" unless klass < ActiveRecord::Base
+    raise TypeError, "association #{association} does not exist on #{klass.name}" unless klass.reflect_on_association(association)
+
+    @klass = klass
+    @association = association
+  end
+
+  def load(model)
+    raise TypeError, "loader for #{klass.name} can't load associations for objects of type #{model.class.name}" unless model.is_a?(klass)
+    model.association(@association).loaded? ? Promise.resolve(model) : super
+  end
+
+  def perform(models)
+    ActiveRecord::Associations::Preloader.new.preload(models, association)
+    models.each { |m| fulfill(m, m) }
+  end
+end
diff --git a/association_preload_instrumentation.rb b/association_preload_instrumentation.rb
@@ -0,0 +1,55 @@
+# frozen_string_literal: true
+class AssociationPreloadInstrumentation
+  def instrument(_type, field)
+    return field unless field.metadata.include?(:preload)
+
+    old_resolver = field.resolve_proc
+    new_resolver = -> (object, args, ctx) do
+      preload(object, field.metadata[:preload]).then do
+        old_resolver.call(object, args, ctx)
+      end
+    end
+
+    field.redefine do
+      resolve(new_resolver)
+    end
+  end
+
+  private
+
+  def preload(object, associations)
+    if associations.is_a?(Symbol)
+      preload_single_association(object, associations)
+    else
+      promises = []
+
+      Array.wrap(associations).each do |value|
+        case value
+        when Symbol
+          promises << preload_single_association(object, value)
+        when Array
+          value.each { |sub_value| promises << preload(object, sub_value) }
+        when Hash
+          value.each do |key, sub_value|
+            promises << preload_single_association(object, key).then do
+              next_value = object.public_send(key)
+              case next_value
+              when ActiveRecord::Base
+                preload(next_value, sub_value)
+              else
+                Promise.all(Array.wrap(next_value).map { |next_model| preload(next_model, sub_value) })
+              end
+            end
+          end
+        end
+      end
+
+      Promise.all(promises)
+    end
+  end
+
+  def preload_single_association(object, association)
+    return Promise.resolve(object) if object.association(association).loaded?
+    AssociationLoader.for(object.class, association).load(object)
+  end
+end
diff --git a/initializer.rb b/initializer.rb
@@ -0,0 +1,8 @@
+# frozen_string_literal: true
+# This goes in a Rails initializer, or some other code that runs before your schema/types are required
+GraphQL::Field.accepts_definitions(
+  preload: -> (type, *args) do
+    type.metadata[:preload] ||= []
+    type.metadata[:preload].concat(args)
+  end
+)
diff --git a/schema.rb b/schema.rb
@@ -0,0 +1,5 @@
+# frozen_string_literal: true
+Schema = GraphQL::Schema.define do
+   # configuration stuff
+  instrument(:field, AssociationPreloadInstrumentation.new)
+end