[9.x] Add whenCounted to JsonResource (#43101)

* Add ability to conditionally include a relation's count

* Pass count into value callback and allow specifying count in relation name

* Add tests

* CS fix

* CS fix

* Remove `hasAttribute` method from `Model`

* Change comparison order to follow suit with other methods

src/Illuminate/Http/Resources/ConditionallyLoadsAttributes.php

@@ -3,6 +3,7 @@
 namespace Illuminate\Http\Resources;
 
 use Illuminate\Support\Arr;
+use Illuminate\Support\Str;
 
 trait ConditionallyLoadsAttributes
 {
@@ -204,6 +205,37 @@ protected function whenLoaded($relationship, $value = null, $default = null)
         return value($value);
     }
 
+    /**
+     * Retrieve a relationship count if it exists.
+     *
+     * @param  string  $relationship
+     * @param  mixed  $value
+     * @param  mixed  $default
+     * @return \Illuminate\Http\Resources\MissingValue|mixed
+     */
+    public function whenCounted($relationship, $value = null, $default = null)
+    {
+        if (func_num_args() < 3) {
+            $default = new MissingValue();
+        }
+
+        $attribute = Str::finish($relationship, '_count');
+
+        if (! isset($this->resource->getAttributes()[$attribute])) {
+            return value($default);
+        }
+
+        if (func_num_args() === 1) {
+            return $this->resource->{$attribute};
+        }
+
+        if ($this->resource->{$attribute} === null) {
+            return;
+        }
+
+        return value($value, $this->resource->{$attribute});
+    }
+
     /**
      * Execute a callback if the given pivot table has been loaded.
      *

tests/Integration/Http/Fixtures/PostResourceWithOptionalRelationshipCounts.php

@@ -0,0 +1,17 @@
+<?php
+
+namespace Illuminate\Tests\Integration\Http\Fixtures;
+
+class PostResourceWithOptionalRelationshipCounts extends PostResource
+{
+    public function toArray($request)
+    {
+        return [
+            'id' => $this->id,
+            'authors' => $this->whenCounted('authors_count'),
+            'comments' => $this->whenCounted('comments', function ($count) {
+                return "$count comments";
+            }, 'None'),
+        ];
+    }
+}

tests/Integration/Http/ResourceTest.php

@@ -33,6 +33,7 @@
 use Illuminate\Tests\Integration\Http\Fixtures\PostResourceWithOptionalMerging;
 use Illuminate\Tests\Integration\Http\Fixtures\PostResourceWithOptionalPivotRelationship;
 use Illuminate\Tests\Integration\Http\Fixtures\PostResourceWithOptionalRelationship;
+use Illuminate\Tests\Integration\Http\Fixtures\PostResourceWithOptionalRelationshipCounts;
 use Illuminate\Tests\Integration\Http\Fixtures\PostResourceWithoutWrap;
 use Illuminate\Tests\Integration\Http\Fixtures\ReallyEmptyPostResource;
 use Illuminate\Tests\Integration\Http\Fixtures\ResourceWithPreservedKeys;
@@ -283,6 +284,59 @@ public function testResourcesMayHaveOptionalRelationships()
         ]);
     }
 
+    public function testResourcesMayHaveOptionalRelationshipCounts()
+    {
+        Route::get('/', function () {
+            $post = new Post([
+                'id' => 5,
+                'title' => 'Test Title',
+            ]);
+
+            return new PostResourceWithOptionalRelationshipCounts($post);
+        });
+
+        $response = $this->withoutExceptionHandling()->get(
+            '/', ['Accept' => 'application/json']
+        );
+
+        $response->assertStatus(200);
+
+        $response->assertExactJson([
+            'data' => [
+                'id' => 5,
+                'comments' => 'None',
+            ],
+        ]);
+    }
+
+    public function testResourcesMayLoadOptionalRelationshipCounts()
+    {
+        Route::get('/', function () {
+            $post = new Post([
+                'id' => 5,
+                'title' => 'Test Title',
+                'authors_count' => 2,
+                'comments_count' => 5,
+            ]);
+
+            return new PostResourceWithOptionalRelationshipCounts($post);
+        });
+
+        $response = $this->withoutExceptionHandling()->get(
+            '/', ['Accept' => 'application/json']
+        );
+
+        $response->assertStatus(200);
+
+        $response->assertExactJson([
+            'data' => [
+                'id' => 5,
+                'authors' => 2,
+                'comments' => '5 comments',
+            ],
+        ]);
+    }
+
     public function testResourcesMayLoadOptionalRelationships()
     {
         Route::get('/', function () {