how to do a mongodb query in a mongodb function?

Question

I'm trying to write a mongo script from the mongo shell, but I'm having a small problem. I'll let the code explain itself.

var shops = db.Shop.find({})

function printShopUrl(data) {
    var name, url;
    for (var i = 0; i < data.length(); i++) {
        name = data[i].name;
        url = db.Instance.findOne({name:name}).url;
        print(url);
    }
}

printShopUrl(shops)

So all i'm trying to do right now is just to print the url, but when I run this query I get an error.

TypeError: db.Instance.findOne({name:name}) has no properties (shell):1

Any idea what i'm doing wrong?

It looks like the query isn't finding anything, in which case it will return null. Print out the intermediate result db.Instance.findOne({name:name}) - what do you see? — YXD
– YXD, Commented Jun 10, 2011 at 1:17
thanks Mr. E, that was exactly the problem i had. i tried to answer my own question, but i didn't have enough point. so i'll post my solution here. .. i guess i can't, it's too long, but yeah, all i did was wrap it in a try catch and it works. — Khon Lieu
– Khon Lieu, Commented Jun 10, 2011 at 1:33
try catch is excessive just do if(result) print(result.url); — YXD
– YXD, Commented Jun 10, 2011 at 1:44

Gates VP · Accepted Answer · 2011-06-12 05:39:16Z

1

Main problem: the following may not return a value db.Instance.findOne({name:name}). Therefore when you add .url, you're trying to get a value from a null.

Try the following:

var obj = db.Instance.findOne({name:name});
if(obj && obj.url) { print(obj.url); }

You have the same potential issue with the name field (name = data[i].name).

answered Jun 12, 2011 at 5:39

Gates VP

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