Add $sortByCount aggregation pipeline stage

lib/Doctrine/ODM/MongoDB/Aggregation/Builder.php

@@ -162,6 +162,14 @@ public function replaceRoot($expression = null)
         return $this->addStage(new Stage\ReplaceRoot($this, $this->dm, $this->class, $expression));
     }
 
+    /**
+     * @return Stage\SortByCount
+     */
+    public function sortByCount($expression)
+    {
+        return $this->addStage(new Stage\SortByCount($this, $expression, $this->dm, $this->class));
+    }
+
     /**
      * {@inheritdoc}
      */

lib/Doctrine/ODM/MongoDB/Aggregation/Stage/SortByCount.php

@@ -0,0 +1,18 @@
+<?php
+
+namespace Doctrine\ODM\MongoDB\Aggregation\Stage;
+
+use Doctrine\MongoDB\Aggregation\Builder;
+use Doctrine\MongoDB\Aggregation\Stage as BaseStage;
+use Doctrine\ODM\MongoDB\DocumentManager;
+use Doctrine\ODM\MongoDB\Mapping\ClassMetadata;
+
+class SortByCount extends BaseStage\SortByCount
+{
+    public function __construct(Builder $builder, $fieldName, DocumentManager $documentManager, ClassMetadata $class)
+    {
+        $documentPersister = $documentManager->getUnitOfWork()->getDocumentPersister($class->name);
+
+        parent::__construct($builder, '$' . $documentPersister->prepareFieldName(substr($fieldName, 1)));
+    }
+}

tests/Doctrine/ODM/MongoDB/Tests/Aggregation/Stage/SortByCountTest.php

@@ -0,0 +1,19 @@
+<?php
+
+namespace Doctrine\ODM\MongoDB\Tests\Aggregation\Stage;
+
+use Documents\CmsComment;
+
+class SortByCountTest extends \Doctrine\ODM\MongoDB\Tests\BaseTest
+{
+    public function testFieldNameConversion()
+    {
+        $builder = $this->dm->createAggregationBuilder(CmsComment::class);
+        $builder->sortByCount('$authorIp');
+
+        $this->assertEquals(
+            [['$sortByCount' => '$ip']],
+            $builder->getPipeline()
+        );
+    }
+}