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Auto Increment with MongoDB

• Thu, 29 Jul 2010 19:52:18 GMT
• 4 Comments
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We are currently working on an app that uses a number of technologies, including PHP, Python, and MongoDB. Recently, a need arose to use sequential identifiers for users, similar to an auto_increment column in MySQL.

First, create a sequence collection that you can use to determine the next identifier in the sequence. The following creates a collection called seq that has a single sequence in it (for users), but you can add as many as you need:

db.seq.insert({"_id":"users", "seq":new NumberLong(1)});

Before adding a new user, you need to increment the sequence by one and fetch the next identifier. Fortunately, the findandmodify() command provides an atomic way to do this. Using the MongoDB shell, the command would look something like this:

db.seq.findAndModify({
    query: {"_id":"users"},
    update: {$inc: {"seq":1}},
    new: true
});

Because I'm using Lithium, I added a method for fetching the next identifier to my User model:

<?php
 
namespace app\models;
 
class User extends \lithium\data\Model {
 
    static public function seq() {
        $seq = static::_connection()->connection->command(
            array('findandmodify' => 'seq',
                  'query' => array('_id' => 'users'),
                  'update' => array('$inc' => array('seq' => 1)),
                  'new' => TRUE
            )
        );
 
        return $seq['value']['seq'];
    }
 
}
 
?>

If you're not using Lithium, you can use the MongoDB class to execute a command().

With this in place, adding a new user is a simple process. I create an array called $data with everything I want to store for a user, and then do the following:

<?php
 
$user = User::create($data);
$user->_id = User::seq();
$success = $user->save();
 
?>

This example should be easy to adapt to any environment. Once you have the next identifier in the sequence, you simply store it as you would any other data.

I hope to blog more about both MongoDB and Lithium. As these technologies are still pretty new to me, please feel free to point out any improvements. I'll update the post accordingly.