Question

My JSON documents (called “i”), have sub documents (called “elements”).
I am looping trhough these subdocuments and updating them one at a time. However, to do so (once the value i need is computed), I have mongo scan through all the documents in the database, then through all the subdocuments, and then find the subdocument it needs to update.

I am having major time issues, as I have ~3000 documents and this is taking about 4minutes.
I would like to know if there is a quicker way to do this, without mongo having to scan all the documents but by doing it within the loop.

Here is the code:

for i in db.stuff.find():

    for element in i['counts']:

        computed_value = element[a] + element[b]
        db.stuff.update({'id':i['id'], 'counts.timestamp':element['timestamp']}, 
                        {'$set': {'counts.$.total':computed_value}})

I am identifying the overall document by “id” and then the subdocument by its timestamp (which is unique to each subdocument). I need to find a quicker way than this. Thank you for your help.

Answer 1

What indexes do you have on your collection ? This could probably be sped up by creating an index on your embedded documents. You can do this using dot notation — there’s a good explanation and example here.

In your case, you’d do something like

db.stuff.ensureIndex( { "i.elements.timestamp" : 1 });

This will make your searches through embedded documents run much faster.