How can I atomically reorder array items in MongoDB?

Question

I have a playlists collection. Each document in the collection contains an entries field, that is an array.

How can I change the order of entries in a playlist, atomically? (e.g. If another user tries at the same time to $push a new entry, I want to make sure that the new entry is $pushed only after the reordering has happened)

If I do a find(), then reorder in PHP, then update(), I might lose the new $pushed entry if the operation is not atomic.

This is a sample playlist object:

{
    "_id" : "playlist1",
    "title" : "Playlist One",
    "entries" : [
            {
                    "class" : "song",
                    "identifier" : "song1"
            },
            {
                    "class" : "song",
                    "identifier" : "song2"
            },
            {
                    "class" : "song",
                    "identifier" : "song3"
            }
    ]
}

---

Edit:

I think I found a possible solution, using execute():

Running JavaScript in the database takes a write lock, meaning it blocks other operations.

(from http://php.net/manual/en/mongodb.execute.php)

I just need to execute the code (find() + reorder() + update()) on the MongoDB Server.

Still, if you have another idea that doesn't involve "code within a string" (e.g. $func = "function() { do_something(); return \"something else\" }"), please share it.

Thanks!

Answer 1

Using eval() to block operations is a side effect of the current implementation, not a feature to rely on.

The MongoDB wiki has a page with information on Atomic Operations.

A relevant strategy for your use case would be "update if current":

1. Fetch the object.
2. Modify the object locally.
3. Send an update request that says "update the object to this new value if it still matches its old value".