PHPLIB-358: Do not require collection name for Aggregate operation

src/Operation/Aggregate.php

@@ -116,8 +116,11 @@ class Aggregate implements Executable
      *    This is not supported for server versions < 3.4 and will result in an
      *    exception at execution time if used.
      *
+     * Note: Collection-agnostic commands (e.g. $currentOp) may be executed by
+     * specifying null for the collection name.
+     *
      * @param string      $databaseName   Database name
-     * @param string $collectionName Collection name
+     * @param string|null $collectionName Collection name
      * @param array       $pipeline       List of pipeline operations
      * @param array       $options        Command options
      * @throws InvalidArgumentException for parameter/option parsing errors
@@ -220,7 +223,7 @@ class Aggregate implements Executable
         }
 
         $this->databaseName = (string) $databaseName;
-        $this->collectionName = (string) $collectionName;
+        $this->collectionName = isset($collectionName) ? (string) $collectionName : null;
         $this->pipeline = $pipeline;
         $this->options = $options;
     }
@@ -289,7 +292,7 @@ class Aggregate implements Executable
     private function createCommand(Server $server)
     {
         $cmd = [
-            'aggregate' => $this->collectionName,
+            'aggregate' => isset($this->collectionName) ? $this->collectionName : 1,
             'pipeline' => $this->pipeline,
         ];
         $cmdOptions = [];

tests/Operation/AggregateFunctionalTest.php

@@ -12,6 +12,28 @@ use stdClass;
 
 class AggregateFunctionalTest extends FunctionalTestCase
 {
+    public function testCurrentOpCommand()
+    {
+        if (version_compare($this->getServerVersion(), '3.6.0', '<')) {
+            $this->markTestSkipped('$currentOp is not supported');
+        }
+
+        (new CommandObserver)->observe(
+            function() {
+                $operation = new Aggregate(
+                    'admin',
+                    null,
+                    [['$currentOp' => (object) []]]
+                );
+
+                $operation->execute($this->getPrimaryServer());
+            },
+            function(stdClass $command) {
+                $this->assertSame(1, $command->aggregate);
+            }
+        );
+    }
+
     public function testDefaultReadConcernIsOmitted()
     {
         (new CommandObserver)->observe(