MongoDB PHP: Insert if not exists equivalent - php

Question

I use MongoDB together with PHP. To create a unique document for each user, I set the username to unique with
$db->ensureIndex(array("username" => 1), array("unique" => 1, "dropDups" => 1));
However, if there is a duplicate, the _id value that is being returned for the user is that of the record which got never created and not that of the record which already exists in the db.
Here's the entire function:
function register_user($username, $password, $first_name, $last_name, $email)
{
$m = new Mongo();
$db = $m->users->details;
$db->ensureIndex(array("username" => 1), array("unique" => 1, "dropDups" => 1));
$user_doc = array( "username" => $username, "password" => $password, "first_name" => $first_name, "last_name" => $last_name, "email" => $email);
$db->insert($user_doc);
return $user_doc["_id"]; // here i return the _id
}
How would I go about creating a user database, that has no duplicate usernames?
EDIT:
Here's my temporary solution. I put it right where the insert used to be.
try
{
$db->insert($user_doc,array('safe'=>1));
}
catch ( MongoCursorException $e )
{
// if this did not work, we already have that user inside and want to get his _id
$user = $db->findOne(array('username' => $username, 'password' => $password));
return $user["_id"];
}
If I try to insert it with the safe option, it would always die. Using the try-catch procedure, I try to insert and if it does not work, manually find the _id by username. I'm still not satisfied with this solution, so it would be great if more proficient MongoDB users could give some advice.

Answer 1

First off, you always need that try...catch. The reason you were getting the wrong _id for $user_doc is simply that the driver creates it before sending it to the database. So even if the insert fails it's still on the document.
How would I go about creating a user database, that has no duplicate usernames?
I would start by over-writing the _id field with the user name. The _id is auto-generated as a convenience, but you can set it yourself before you insert.
You will still need to use the try...catch but you will get a specific duplicate key error. You can then use this to correctly handle the case of "that user name already exists".

Answer 2

Here's how I manage to insert if not exists.
$db->update(
array('email' => $email),
array('$setOnInsert' => array('email' => $email)),
array('upsert' => true)
);
This basically just updates when the record exists ("updating" to what it's already set to and $setOnInsert does nothing), but if it doesn't exist, mongo upserts a new record then creates the new key and assigns the value.