MongoDB and complex query on ordered arrays

Question

I've the following entity persisted on mongo in a users collection:

{
    "fullname": "Luke Tomson",
    "nicknames": [
        {
            "name": "lukeone",
            "created": ISODate("2011-05-18T19:42:56.411Z")
        },
        {
            "name": "magicluke",
            "created": ISODate("2012-01-12T19: 42: 56.411Z")
        },
        {
            "name": "superluke",
            "created": ISODate("2012-03-27T19: 42: 56.411Z")
        }
    }
]
}

How do I get the last nickname name for a user whose fullname is "Luke Tomson" ? And more specifically, how do I retrieve in the mongo shell a specific index (the first or the last) of a sorted array that was sorted upon a specific field (in this case, created).

Thanks

user1207456 · Accepted Answer · 2012-03-27 20:14:15Z

1

From the documentation, the second object passed to a .find is a selection of what portions of the object you want returned.

So, what you want is basically:

db.users.find({"fullname": "Luke Tomson"}, {"nicknames": { $slice: -1 } });

Though you might also want to return the _id if there are multiple "Luke Tomson"s in your DB and you need to uniquely identify them.

answered Mar 27, 2012 at 20:14

user1207456

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3 Comments

Mark Over a year ago

And is there a way to actually retrieve the user who's last nickname is "superluke" ?

user1207456 Over a year ago

If you query an array element with a non-array value, it will return all records where at least one of the records contains the specified value. If you really need only the records where superluke is the last nickname, then you can fall back to the $where table-scan-style query. The string you pass to $where should be a single Javascript statement that evaluates to a boolean and this is a single MongoDB record. db.users.find({$where: "this.nicknames[this.nicknames.length-1].name == 'superluke'"}). You may be able to chain finds to reduce the scan size.

user1207456 Over a year ago

Oh, by the way, this page is your friend. It's very dense, but I got everything I know about MongoDB from either this page or the other I linked to you. (And experience playing with the node-mongodb-native library.)

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