Dataloader support

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## master
 
+- [PR#130](https://github.com/DmitryTsepelev/graphql-ruby-fragment_cache/pull/130) Dataloader support ([@DmitryTsepelev][])
 - [PR#125](https://github.com/DmitryTsepelev/graphql-ruby-fragment_cache/pull/125) Introduce cache lookup instrumentation hook ([@danielhartnell][])
 
 ## 1.20.5 (2024-11-02)

README.md

@@ -381,6 +381,34 @@ class QueryType < BaseObject
 end
 ```
 
+## Dataloader
+
+If you are using [Dataloader](https://graphql-ruby.org/dataloader/overview.html), you will need to let the gem know using `dataloader: true`:
+
+```ruby
+class PostType < BaseObject
+  field :author, User, null: false
+
+  def author
+    cache_fragment(dataloader: true) do
+      dataloader.with(AuthorDataloaderSource).load(object.id)
+    end
+  end
+end
+
+# or
+
+class PostType < BaseObject
+  field :author, User, null: false, cache_fragment: {dataloader: true}
+
+  def author
+    dataloader.with(AuthorDataloaderSource).load(object.id)
+  end
+end
+```
+
+The problem is that I didn't find a way to detect that dataloader (and, therefore, Fiber) is used, and the block is forced to resolve, causing the N+1 inside the Dataloader Source class.
+
 ## How to use `#cache_fragment` in extensions (and other places where context is not available)
 
 If you want to call `#cache_fragment` from places other that fields or resolvers, you'll need to pass `context` explicitly and turn on `raw_value` support. For instance, let's take a look at this extension:

lib/graphql/fragment_cache/object_helpers.rb

@@ -48,6 +48,11 @@ def cache_fragment(object_to_cache = NO_OBJECT, **options, &block)
 
         fragment = Fragment.new(context_to_use, **options)
 
+        if options.delete(:dataloader)
+          object_to_cache = block.call
+          block = nil
+        end
+
         GraphQL::FragmentCache::Schema::LazyCacheResolver.new(fragment, context_to_use, object_to_cache, &block)
       end
     end

spec/graphql/fragment_cache/object_helpers_spec.rb

@@ -777,6 +777,59 @@ def post(id:, expires_in: nil)
     end
   end
 
+  describe "caching fields with dataloader" do
+    let(:query) do
+      <<~GQL
+        query GetPosts {
+          posts {
+            id
+            dataloaderCachedAuthor {
+              name
+            }
+          }
+        }
+      GQL
+    end
+
+    let(:schema) do
+      build_schema do
+        use GraphQL::Dataloader
+        query(Types::Query)
+      end
+    end
+
+    let(:user1) { User.new(id: 1, name: "User #1") }
+    let(:user2) { User.new(id: 2, name: "User #2") }
+
+    let!(:post1) { Post.create(id: 1, title: "object test 1", author: user1) }
+    let!(:post2) { Post.create(id: 2, title: "object test 2", author: user2) }
+
+    before do
+      allow(User).to receive(:find_by_post_ids).and_call_original
+
+      # warmup cache
+      execute_query
+      expect(User).to have_received(:find_by_post_ids).with([post1.id, post2.id])
+
+      # make objects dirty
+      user1.name = "User #1 new"
+      user2.name = "User #2 new"
+    end
+
+    it "returns cached results" do
+      expect(execute_query.dig("data", "posts")).to eq([
+        {
+          "id" => "1",
+          "dataloaderCachedAuthor" => {"name" => "User #1"}
+        },
+        {
+          "id" => "2",
+          "dataloaderCachedAuthor" => {"name" => "User #2"}
+        }
+      ])
+    end
+  end
+
   describe "conditional caching" do
     let(:schema) do
       field_resolver = resolver

spec/support/models/user.rb

@@ -4,6 +4,12 @@ class User
   attr_reader :id
   attr_accessor :name
 
+  class << self
+    def find_by_post_ids(post_ids)
+      post_ids.map { |id| Post.find(id).author }
+    end
+  end
+
   def initialize(id:, name:)
     @id = id
     @name = name

spec/support/test_schema.rb

@@ -7,6 +7,12 @@ def perform(posts)
   end
 end
 
+class AuthorDataloaderSource < GraphQL::Dataloader::Source
+  def fetch(post_ids)
+    User.find_by_post_ids(post_ids)
+  end
+end
+
 module Types
   class Base < GraphQL::Schema::Object
     include GraphQL::FragmentCache::Object
@@ -41,6 +47,7 @@ class Post < Base
     field :cached_author, User, null: false
     field :batched_cached_author, User, null: false
     field :cached_author_inside_batch, User, null: false
+    field :dataloader_cached_author, User, null: false
 
     field :meta, String, null: true
 
@@ -60,6 +67,12 @@ def cached_author_inside_batch
         cache_fragment(author, context: context)
       end
     end
+
+    def dataloader_cached_author
+      cache_fragment(dataloader: true) do
+        dataloader.with(AuthorDataloaderSource).load(object.id)
+      end
+    end
   end
 
   class PostInput < GraphQL::Schema::InputObject