add sub document aggregation array functionality

example:
    Model::sum('subarray.*.price');

this method is much simpler as to use as complex raw aggregations

for this function a $unwind directive will be pushed in pipeline before $group

Author: Fiete Börner

src/Jenssegers/Mongodb/Query/Builder.php

@@ -179,6 +179,7 @@ public function getFresh($columns = [])
         // Use MongoDB's aggregation framework when using grouping or aggregation functions.
         if ($this->groups or $this->aggregate or $this->paginating) {
             $group = [];
+            $unwinds = [];
 
             // Add grouping columns to the $group part of the aggregation pipeline.
             if ($this->groups) {
@@ -204,6 +205,13 @@ public function getFresh($columns = [])
                 $function = $this->aggregate['function'];
 
                 foreach ($this->aggregate['columns'] as $column) {
+                    // Add unwind if a subdocument array should be aggregated
+                    // column: subarray.price => {$unwind: '$subarray'}
+                    if (count($splitColumns = explode('.*.', $column)) == 2) {
+                        $unwinds[] = $splitColumns[0];
+                        $column = implode('.', $splitColumns);
+                    }
+
                     // Translate count into sum.
                     if ($function == 'count') {
                         $group['aggregate'] = ['$sum' => 1];
@@ -233,6 +241,12 @@ public function getFresh($columns = [])
             if ($wheres) {
                 $pipeline[] = ['$match' => $wheres];
             }
+
+            // apply unwinds for subdocument array aggregation
+            foreach($unwinds as $unwind){
+                $pipeline[] = ['$unwind' => '$' . $unwind];
+            }
+
             if ($group) {
                 $pipeline[] = ['$group' => $group];
             }

tests/QueryBuilderTest.php

@@ -420,6 +420,22 @@ public function testSubdocumentAggregate()
         $this->assertEquals(16.25, DB::collection('items')->avg('amount.hidden'));
     }
 
+    public function testSubdocumentArrayAggregate()
+    {
+        DB::collection('items')->insert([
+            ['name' => 'knife', 'amount' => [['hidden' => 10, 'found' => 3]]],
+            ['name' => 'fork',  'amount' => [['hidden' => 35, 'found' => 12]]],
+            ['name' => 'spoon', 'amount' => [['hidden' => 14, 'found' => 21]]],
+            ['name' => 'spoon', 'amount' => [['hidden' => 6, 'found' => 4]]],
+        ]);
+
+        $this->assertEquals(65, DB::collection('items')->sum('amount.*.hidden'));
+        $this->assertEquals(4, DB::collection('items')->count('amount.*.hidden'));
+        $this->assertEquals(6, DB::collection('items')->min('amount.*.hidden'));
+        $this->assertEquals(35, DB::collection('items')->max('amount.*.hidden'));
+        $this->assertEquals(16.25, DB::collection('items')->avg('amount.*.hidden'));
+    }
+
     public function testUpsert()
     {
         DB::collection('items')->where('name', 'knife')