app/Models/Item.php: add reagent_for relationship.

tests/Feature/Models/ItemTest.php

@@ -33,6 +33,27 @@ class ItemTest extends TestCase
         $this->assertEquals($recipe2->id, $item->recipes[1]->id);
     }
 
+    /**
+     * Test reagent for relationship.
+     *
+     * @return void
+     */
+    public function test_reagent_for_relationship()
+    {
+        $item = Item::factory()->create();
+
+        $recipe = Recipe::factory()
+            ->hasAttached($item, ['quantity' => 1], 'reagents')
+            ->create();
+
+        $recipe2 = Recipe::factory()
+            ->hasAttached($item, ['quantity' => 1], 'reagents')
+            ->create();
+
+        $this->assertEquals($recipe->id, $item->reagent_for[0]->id);
+        $this->assertEquals($recipe2->id, $item->reagent_for[1]->id);
+    }
+
     /**
      * Test quantity attribute
      *