Add $unionWith stage to aggregation pipeline builder

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

@@ -629,6 +629,21 @@ public function sortByCount(string $expression): Stage\SortByCount
         return $stage;
     }
 
+    /**
+     * Performs a union of two collections. $unionWith combines pipeline results
+     * from two collections into a single result set. The stage outputs the
+     * combined result set (including duplicates) to the next stage.
+     *
+     * @see https://www.mongodb.com/docs/rapid/reference/operator/aggregation/unionWith/
+     */
+    public function unionWith(string $collection): Stage\UnionWith
+    {
+        $stage = new Stage\UnionWith($this, $this->dm, $collection);
+        $this->addStage($stage);
+
+        return $stage;
+    }
+
     /**
      * Removes/excludes fields from documents.
      *

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

@@ -412,6 +412,18 @@ public function sort($fieldName, $order = null): Stage\Sort
         return $this->builder->sort($fieldName, $order);
     }
 
+    /**
+     * Performs a union of two collections. $unionWith combines pipeline results
+     * from two collections into a single result set. The stage outputs the
+     * combined result set (including duplicates) to the next stage.
+     *
+     * @see https://www.mongodb.com/docs/rapid/reference/operator/aggregation/unionWith/
+     */
+    public function unionWith(string $collection): Stage\UnionWith
+    {
+        return $this->builder->unionWith($collection);
+    }
+
     /**
      * Removes/excludes fields from documents.
      *

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

@@ -0,0 +1,75 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Doctrine\ODM\MongoDB\Aggregation\Stage;
+
+use Doctrine\ODM\MongoDB\Aggregation\Builder;
+use Doctrine\ODM\MongoDB\Aggregation\Stage;
+use Doctrine\ODM\MongoDB\DocumentManager;
+use Doctrine\Persistence\Mapping\MappingException;
+use InvalidArgumentException;
+
+/**
+ * Fluent interface for adding a $unionWith stage to an aggregation pipeline.
+ *
+ * @psalm-type Pipeline = Builder|Stage|list<array<string, mixed>>
+ */
+class UnionWith extends Stage
+{
+    private DocumentManager $dm;
+
+    private string $collection;
+
+    /**
+     * @var array|Builder|null
+     * @psalm-var Pipeline|null
+     */
+    private $pipeline = null;
+
+    public function __construct(Builder $builder, DocumentManager $documentManager, string $collection)
+    {
+        parent::__construct($builder);
+
+        $this->dm = $documentManager;
+
+        try {
+            $class            = $this->dm->getClassMetadata($collection);
+            $this->collection = $class->getCollection();
+        } catch (MappingException $e) {
+            $this->collection = $collection;
+        }
+    }
+
+    /**
+     * @param array|Builder|Stage $pipeline
+     * @psalm-param Pipeline $pipeline
+     */
+    public function pipeline($pipeline): self
+    {
+        if ($pipeline instanceof Stage) {
+            $this->pipeline = $pipeline->builder;
+        } else {
+            $this->pipeline = $pipeline;
+        }
+
+        if ($this->builder === $this->pipeline) {
+            throw new InvalidArgumentException('Cannot use the same Builder instance for $unionWith pipeline.');
+        }
+
+        return $this;
+    }
+
+    public function getExpression(): array
+    {
+        $params = (object) ['coll' => $this->collection];
+
+        if ($this->pipeline) {
+            $params->pipeline = $this->pipeline instanceof Builder
+                ? $this->pipeline->getPipeline(false)
+                : $this->pipeline;
+        }
+
+        return ['$unionWith' => $params];
+    }
+}

phpstan-baseline.neon

@@ -157,6 +157,7 @@ parameters:
                 - lib/Doctrine/ODM/MongoDB/Aggregation/Stage/Lookup.php
                 - lib/Doctrine/ODM/MongoDB/Aggregation/Stage/Merge.php
                 - lib/Doctrine/ODM/MongoDB/Aggregation/Stage/Out.php
+                - lib/Doctrine/ODM/MongoDB/Aggregation/Stage/UnionWith.php
 
         # $this->mapping['targetDocument'] is class-string<T>
         -

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

@@ -0,0 +1,50 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Doctrine\ODM\MongoDB\Tests\Aggregation\Stage;
+
+use Doctrine\ODM\MongoDB\Tests\BaseTest;
+use Documents\SimpleReferenceUser;
+use Documents\User;
+
+class UnionWithTest extends BaseTest
+{
+    public function testUnionWithStageWithClassName(): void
+    {
+        $builder = $this->dm->createAggregationBuilder(SimpleReferenceUser::class);
+        $builder
+            ->unionWith(User::class);
+
+        $expectedPipeline = [
+            ['$unionWith' => (object) ['coll' => 'users']],
+        ];
+
+        self::assertEquals($expectedPipeline, $builder->getPipeline());
+    }
+
+    public function testUnionWithStageWithCollectionName(): void
+    {
+        $unionBuilder = $this->dm->createAggregationBuilder(SimpleReferenceUser::class);
+        $unionBuilder->match()
+            ->field('foo')->equals('bar');
+
+        $builder = $this->dm->createAggregationBuilder(SimpleReferenceUser::class);
+        $builder
+            ->unionWith('someRandomCollectionName')
+                ->pipeline($unionBuilder);
+
+        $expectedPipeline = [
+            [
+                '$unionWith' => (object) [
+                    'coll' => 'someRandomCollectionName',
+                    'pipeline' => [
+                        ['$match' => ['foo' => 'bar']],
+                    ],
+                ],
+            ],
+        ];
+
+        self::assertEquals($expectedPipeline, $builder->getPipeline());
+    }
+}