public function testPreserveId()
{
$document = MockMongoPost::create(array('_id' => 'custom'), array('exists' => true));
array_push($this->_db->connection->results, true);
$this->assertTrue($document->save(array('_id' => 'custom2', 'foo' => 'bar')));
$result = array_shift($this->_db->connection->queries);
$expected = array('$set' => array('foo' => 'bar'));
$this->assertEqual($expected, $result['update']);
}