New MongoDB Drivers for PHP and HHVM: Cursor Behaviour

We released a new version of the MongoDB driver for PHP (the mongodb extension) near the end of last year. In a previous blog post, I covered the back story of the how and why we undertook this effort. And in another post, I spoke about the architecture of the new driver. In this episode I will discuss changes in cursor behaviour.

I recently found the following comment added to the driver's documentation, on, which read:

I noticed that ->sort is missing from the cursor. Seems like the old driver has more functionality.

The new driver certainly allows for sorting of results of queries, but no longer by calling the sort() method on the cursor object. This is because cursors are now created differently than in the old mongo extension.

Legacy Driver

In the old driver, the cursor would not really be created until after the first rewind() call on the iterator:

$m = new MongoClient;

// Select 'demo' database and 'example' collection
$collection = $m->demo->example;

// Create the cursor
$cursor = $collection->find();

At this moment, although a cursor object had been created, the query had not yet executed (i.e. it was not sent to the server). The query would only be executed by starting iteration with foreach ( $cursor as $result ) or calling $cursor->rewind(). This gives you the chance to configure the cursor's query with sort(), limit(), and skip() before it is executed by the server:

// Add sort, and limit
$cursor->sort( [ 'name' => 1 ] )->limit( 40 );

After the cursor starts iterating (through foreach or ->rewind()), you can no longer call the aforementioned methods, as well as other methods that configure the query, to modify the cursor.

New Driver

In the new driver, as soon as you have a \MongoDB\Driver\Cursor object, it has already been processed by the server. Because sort (and limit and skip) parameters need to be sent to the server before the query is executed, you can not retroactively call them on an existing Cursor object.

You can use sort (and limit and skip) with the new driver, but they must be specified as options to the \MongoDB\Driver\Query object before it is passed to \MongoDB\Driver\Manager::executeQuery():

$m = new \MongoDB\Driver\Manager();
$ns = 'demo.example';

// Create query object with all options:
$query = new \MongoDB\Driver\Query(
        [], // query (empty: select all)
        [ 'sort' => [ 'name' => 1 ], 'limit' => 40 ] // options

// Execute query and obtain cursor:
$cursor = $manager->executeQuery( $ns, $query );

What's Next?

In the next installment of this series on the new MongoDB driver, we will focus on a new feature in the new MongoDB driver: ODS - the Object Data Store.


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