[8.x] Add withOnly method (#37144)

* Added in new withOnly method + tests

* Refactored function

* Update Builder.php

Co-authored-by: Taylor Otwell <taylor@laravel.com>

src/Illuminate/Database/Eloquent/Builder.php

@@ -1236,6 +1236,19 @@ public function without($relations)
         return $this;
     }
 
+    /**
+     * Set the relationships that should be eager loaded while removing any previously added eager loading specifications.
+     *
+     * @param  mixed  $relations
+     * @return $this
+     */
+    public function withOnly($relations)
+    {
+        $this->eagerLoad = [];
+
+        return $this->with($relations);
+    }
+
     /**
      * Create a new instance of the model being queried.
      *

tests/Database/DatabaseEloquentModelTest.php

@@ -327,6 +327,15 @@ public function testWithoutMethodRemovesEagerLoadedRelationshipCorrectly()
         $this->assertEmpty($instance->getEagerLoads());
     }
 
+    public function testWithOnlyMethodLoadsRelationshipCorrectly()
+    {
+        $model = new EloquentModelWithoutRelationStub();
+        $this->addMockConnection($model);
+        $instance = $model->newInstance()->newQuery()->withOnly('taylor');
+        $this->assertNotNull($instance->getEagerLoads()['taylor']);
+        $this->assertArrayNotHasKey('foo', $instance->getEagerLoads());
+    }
+
     public function testEagerLoadingWithColumns()
     {
         $model = new EloquentModelWithoutRelationStub;