Fix loading for nested through associations

lib/dataloader/ecto.ex

@@ -326,7 +326,11 @@ if Code.ensure_loaded?(Ecto) do
       defp chase_down_queryable([field | fields], schema) do
         case schema.__schema__(:association, field) do
           %{queryable: queryable} ->
-            chase_down_queryable(fields, queryable)
+           chase_down_queryable(fields, queryable)
+
+          %Ecto.Association.HasThrough{through: [through_field | through_fields]} ->
+            [through_field | through_fields ++ fields]
+            |> chase_down_queryable(schema)
         end
       end
 

priv/test_repo/migrations/1_migrate_all.exs

@@ -2,8 +2,13 @@ defmodule Absinthe.Ecto.TestRepo.Migrations.MigrateAll do
   use Ecto.Migration
 
   def change do
+    create table(:leaderboards) do
+      add :name, :string
+    end
+
     create table(:users) do
       add :username, :string
+      add :leaderboard_id, references(:leaderboards)
     end
 
     create table(:posts) do
@@ -16,5 +21,10 @@ defmodule Absinthe.Ecto.TestRepo.Migrations.MigrateAll do
       add :user_id, references(:users), null: false
       add :post_id, references(:posts), null: false
     end
+
+    create table(:scores) do
+      add :leaderboard_id, references(:leaderboards), null: false
+      add :post_id, references(:posts), null: false
+    end
   end
 end

test/dataloader/ecto_test.exs

@@ -1,7 +1,7 @@
 defmodule Dataloader.EctoTest do
   use ExUnit.Case, async: true
 
-  alias Dataloader.{User, Post, Like}
+  alias Dataloader.{User, Post, Like, Score, Leaderboard}
   import Ecto.Query
   alias Dataloader.TestRepo, as: Repo
 
@@ -291,6 +291,33 @@ defmodule Dataloader.EctoTest do
     assert length(loaded_posts) == 2
   end
 
+  test "works with nested has many through", %{loader: loader} do
+    leaderboard = %Leaderboard{name: "Bestliked"} |> Repo.insert!()
+    user = %User{username: "Ben Wilson", leaderboard_id: leaderboard.id} |> Repo.insert!()
+
+    post = %Post{user_id: user.id} |> Repo.insert!()
+    _score = %Score{post_id: post.id, leaderboard_id: leaderboard.id} |> Repo.insert!()
+    _like1 = %Like{post_id: post.id, user_id: user.id} |> Repo.insert!()
+    _like2 = %Like{post_id: post.id, user_id: user.id} |> Repo.insert!()
+
+    loader =
+      loader
+      |> Dataloader.load(Test, :awarded_posts, user)
+      |> Dataloader.load(Test, :likes, user)
+      |> Dataloader.run()
+
+    loaded_posts =
+      loader
+      |> Dataloader.get(Test, :awarded_posts, user)
+    loaded_likes =
+      loader
+      |> Dataloader.get(Test, :likes, user)
+
+
+    assert length(loaded_posts) == 1
+    assert length(loaded_likes) == 2
+  end
+
   test "preloads aren't used", %{loader: loader} do
     user = %User{username: "Ben Wilson"} |> Repo.insert!()
 

test/support/leaderboard.ex

@@ -0,0 +1,8 @@
+defmodule Dataloader.Leaderboard do
+  use Ecto.Schema
+
+  schema "leaderboards" do
+    field(:name, :string)
+    has_many(:scores, Dataloader.Score)
+  end
+end

test/support/score.ex

@@ -0,0 +1,8 @@
+defmodule Dataloader.Score do
+  use Ecto.Schema
+
+  schema "scores" do
+    belongs_to(:post, Dataloader.Post)
+    belongs_to(:leaderboard, Dataloader.Leaderboard)
+  end
+end

test/support/user.ex

@@ -4,5 +4,10 @@ defmodule Dataloader.User do
   schema "users" do
     field(:username, :string)
     has_many(:posts, Dataloader.Post)
+    belongs_to(:leaderboard, Dataloader.Leaderboard)
+
+    has_many(:scores, through: [:leaderboard, :scores])
+    has_many(:awarded_posts, through: [:scores, :post])
+    has_many(:likes, through: [:awarded_posts, :likes])
   end
 end