Update MongoDB subdocument field using value of another field

Question

Based on this SO question and actually this answer I try to update a field on a subdocument.

My code is:

db.venue.find().limit(10).forEach( 
    function (elem) {
        for (var i = elem.events.length - 1; i >= 0; i--) {
            elem.events[i].urls = [];            
            if(elem.events[i].facebook_id){
                elem.events[i].urls = {'facebook_url' : elem.events[i].website }
            }
            if(elem.events[i].eventbrite_id){
                elem.events[i].urls = {'eventbrite_url' : elem.events[i].website }
            }
            //delete elem.events[i].website
        };        
        db.venue.update(elem);
    }
);

If the elem.events[i] has a facebook_id or eventbrite_id update respectively the urls which is an array (for schema reasons).

However I get a

Error: Printing Stack Trace
    at printStackTrace (src/mongo/shell/utils.js:37:15)
    at doassert (src/mongo/shell/assert.js:6:5)
    at assert (src/mongo/shell/assert.js:14:5)
    at DBCollection.update (src/mongo/shell/collection.js:220:5)
    at (shell):13:18
    at DBQuery.forEach (src/mongo/shell/query.js:264:9)
    at (shell):1:27
Mon Jan  6 20:10:48.758 assert failed : need an object at src/mongo/shell/assert.js:7

Which I cannot really understand. Current user's privileges are fine.

Answer 1

You need to call save, not update:

`db.venue.save(elem);`

Update takes two parameters: query document, and an update document.

The mistake is at the origin post you had linked.