PHPLIB-376: countDocuments() throws an exception if no documents are found

src/Operation/CountDocuments.php

@@ -152,8 +152,15 @@ class CountDocuments implements Executable
         }
 
         $cursor = $server->executeReadCommand($this->databaseName, new Command($this->createCommandDocument()), $this->createOptions());
-        $result = current($cursor->toArray());
+        $allResults = $cursor->toArray();
 
+        /* If there are no documents to count, the aggregation pipeline has no items to group, and
+         * hence the result is an empty array (PHPLIB-376) */
+        if (count($allResults) == 0) {
+            return 0;
+        }
+
+        $result = current($allResults);
         if ( ! isset($result->n) || ! (is_integer($result->n) || is_float($result->n))) {
             throw new UnexpectedValueException('count command did not return a numeric "n" value');
         }

tests/Operation/CountDocumentsFunctionalTest.php

+<?php
+
+namespace MongoDB\Tests\Operation;
+
+use MongoDB\Operation\CountDocuments;
+use MongoDB\Operation\InsertMany;
+use stdClass;
+
+class CountDocumentsFunctionalTest extends FunctionalTestCase
+{
+    public function testEmptyCollection()
+    {
+        $operation = new CountDocuments($this->getDatabaseName(), $this->getCollectionName(), []);
+        $this->assertSame(0, $operation->execute($this->getPrimaryServer()));
+    }
+
+    public function testNonEmptyCollection()
+    {
+        $insertMany = new InsertMany($this->getDatabaseName(), $this->getCollectionName(), [
+            ['x' => 1],
+            ['x' => 2],
+            ['y' => 3],
+            ['z' => 4],
+        ]);
+        $insertMany->execute($this->getPrimaryServer());
+
+        $operation = new CountDocuments($this->getDatabaseName(), $this->getCollectionName(), []);
+        $this->assertSame(4, $operation->execute($this->getPrimaryServer()));
+    }
+}