[5.8] Add support for typed eager loads

src/Illuminate/Database/Eloquent/Relations/MorphTo.php

@@ -37,6 +37,13 @@ class MorphTo extends BelongsTo
      */
     protected $macroBuffer = [];
 
+    /**
+     * A map of relations to load for each individual morph type.
+     *
+     * @var array
+     */
+    protected $typedEagerLoads = [];
+
     /**
      * Create a new morph to relationship instance.
      *
@@ -111,7 +118,10 @@ protected function getResultsByType($type)
 
         $query = $this->replayMacros($instance->newQuery())
                             ->mergeConstraintsFrom($this->getQuery())
-                            ->with($this->getQuery()->getEagerLoads());
+                            ->with(array_merge(
+                                $this->getQuery()->getEagerLoads(),
+                                $this->typedEagerLoads[get_class($instance)] ?? []
+                            ));
 
         return $query->whereIn(
             $instance->getTable().'.'.$ownerKey, $this->gatherKeysByType($type)
@@ -253,6 +263,24 @@ public function getDictionary()
         return $this->dictionary;
     }
 
+    /**
+     * Specify which relations to load for a given morph type.
+     *
+     * @param string $modelClass
+     * @param array $with
+     *
+     * @return \Illuminate\Database\Eloquent\Relations\MorphTo
+     */
+    public function withMorph(string $modelClass, array $with): self
+    {
+        $this->typedEagerLoads[$modelClass] = array_merge(
+            $this->typedEagerLoads[$modelClass] ?? [],
+            $with
+        );
+
+        return $this;
+    }
+
     /**
      * Replay stored macro calls on the actual related instance.
      *

tests/Integration/Database/EloquentMorphEagerLoadingTest.php

@@ -0,0 +1,94 @@
+<?php
+
+namespace Illuminate\Tests\Integration\Database\EloquentMorphToLazyEagerLoadingTest;
+
+use DB;
+use Illuminate\Database\Eloquent\Relations\MorphTo;
+use Illuminate\Support\Facades\Schema;
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Tests\Integration\Database\DatabaseTestCase;
+
+/**
+ * @group integration
+ */
+class EloquentMorphToLazyEagerLoadingTest extends DatabaseTestCase
+{
+    protected function setUp(): void
+    {
+        parent::setUp();
+
+        Schema::create('users', function (Blueprint $table) {
+            $table->increments('id');
+        });
+
+        Schema::create('posts', function (Blueprint $table) {
+            $table->increments('post_id');
+            $table->unsignedInteger('user_id');
+        });
+
+        Schema::create('videos', function (Blueprint $table) {
+            $table->increments('video_id');
+        });
+
+        Schema::create('comments', function (Blueprint $table) {
+            $table->increments('id');
+            $table->string('commentable_type');
+            $table->integer('commentable_id');
+        });
+
+        $user = User::create();
+
+        $post = tap((new Post)->user()->associate($user))->save();
+
+        $video = Video::create();
+
+        (new Comment)->commentable()->associate($post)->save();
+        (new Comment)->commentable()->associate($video)->save();
+    }
+
+    public function test_with_morph_loading()
+    {
+        $comments = Comment::query()
+            ->with(['commentable' => function (MorphTo $morphTo) {
+                $morphTo->withMorph(Post::class, ['user']);
+            }])
+            ->get();
+
+        $this->assertTrue($comments[0]->relationLoaded('commentable'));
+        $this->assertTrue($comments[0]->commentable->relationLoaded('user'));
+        $this->assertTrue($comments[1]->relationLoaded('commentable'));
+    }
+}
+
+class Comment extends Model
+{
+    public $timestamps = false;
+
+    public function commentable()
+    {
+        return $this->morphTo();
+    }
+}
+
+class Post extends Model
+{
+    public $timestamps = false;
+    protected $primaryKey = 'post_id';
+
+    public function user()
+    {
+        return $this->belongsTo(User::class);
+    }
+}
+
+class User extends Model
+{
+    public $timestamps = false;
+}
+
+class Video extends Model
+{
+    public $timestamps = false;
+    protected $primaryKey = 'video_id';
+}