This is a proposal for a new feature that makes it possible to limit a result to only one object.
The limit is not applied via an standard sorting, but using a conditional operator with a list of parameters in decreasing priority.
Conditional operators like $in and $or will return only the first matched object.
The object can be a document or an array item.

The use case: multilanguage documents with support for fallback to a default language.

For this document:

{
    "_id": 1,
    "author": "John",
    "languages": [
        {
            "code": "en",
            "data": {
                "title": "Title",
                "contents": "My text."
            }
        },
        {
            "code": "fr",
            "data": {
                "title": "Le titre",
                "contents": "Mon texte."
            }
        },
        {
            "code": "it",
            "data": {
                "title": "Titolo",
                "contents": "Il mio testo."
            }
        }
    ]
}

I'd like to query conditionally for the user (website visitor) language, with a fallback to the default (app) language, but returning only one of them.
For that, I'd propose the new conditional operators $inFirst and $orFirst. They would return only the first match, evaluated from the left to the right.
I'll use $inFirst in the example below.

Suppose user's language is 'fr' and app default language is 'it'.

db.col.find( { _id:1, 'languages.code': { $inFirst: [ 'fr', 'it' ] } }, { author: 1, 'languages.$': 1 } )

Please note that the query features a (hopefully) future feature specified in SERVER-828.

Expected result:

{
    "_id": 1,
    "author": "John",
    "languages": [
        {
            "code": "fr",
            "data": {
                "title": "Le titre",
                "contents": "Mon texte."
            }
        }
    ]
}

I hope that this can be implemented without issues with indexes, parallel processing and sharded setups.