Open up aggregation builder API for use in ODM

lib/Doctrine/MongoDB/Aggregation/Builder.php

@@ -303,7 +303,7 @@ public function unwind($fieldName)
      * @param Stage $stage
      * @return Stage
      */
-    private function addStage(Stage $stage)
+    protected function addStage(Stage $stage)
     {
         $this->stages[] = $stage;
 

lib/Doctrine/MongoDB/Aggregation/Expr.php

@@ -286,7 +286,7 @@ public function cond($if, $then, $else)
      * @param mixed|self $expression
      * @return mixed
      */
-    private function ensureArray($expression)
+    protected function ensureArray($expression)
     {
         if (is_array($expression)) {
             $array = [];

lib/Doctrine/MongoDB/Aggregation/Stage.php

@@ -33,7 +33,7 @@ abstract class Stage
     /**
      * @var Builder
      */
-    private $builder;
+    protected $builder;
 
     /**
      * @param Builder $builder

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

@@ -19,8 +19,8 @@
 
 namespace Doctrine\MongoDB\Aggregation\Stage;
 
+use Doctrine\MongoDB\Aggregation\Builder;
 use Doctrine\MongoDB\Aggregation\Expr;
-use Doctrine\MongoDB\Aggregation\Stage;
 
 /**
  * Fluent interface for adding a $group stage to an aggregation pipeline.
@@ -30,6 +30,21 @@
  */
 class Group extends Operator
 {
+    /**
+     * @var Expr
+     */
+    protected $expr;
+
+    /**
+     * {@inheritdoc}
+     */
+    public function __construct(Builder $builder)
+    {
+        parent::__construct($builder);
+
+        $this->expr = $builder->expr();
+    }
+
     /**
      * {@inheritdoc}
      */

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

@@ -44,7 +44,7 @@ public function __construct(Builder $builder)
     {
         parent::__construct($builder);
 
-        $this->query = new Expr();
+        $this->query = $this->expr();
     }
 
     /**
@@ -196,7 +196,7 @@ public function exists($bool)
      */
     public function expr()
     {
-        return new Expr();
+        return $this->builder->matchExpr();
     }
 
     /**

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

@@ -41,9 +41,9 @@ abstract class Operator extends Stage
      */
     public function __construct(Builder $builder)
     {
-        $this->expr = new Expr();
-
         parent::__construct($builder);
+
+        $this->expr = $builder->expr();
     }
 
     /**

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

@@ -43,7 +43,7 @@ public function __construct(Builder $builder, $collection)
     {
         parent::__construct($builder);
 
-        $this->collection = $collection;
+        $this->out($collection);
     }
 
     /**

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

@@ -18,6 +18,7 @@
  */
 
 namespace Doctrine\MongoDB\Aggregation\Stage;
+
 use Doctrine\MongoDB\Aggregation\Expr;
 
 /**