Updating a number of objects in a nested array

Question

Lets say we have this document:

{
    "_id" : ObjectId("4faaba123412d654fe83hg876"),
    "items" : [
            {
                    "name" : "item_one",
                    "bought" : true
            },
            {
                    "name" : "my_item",
                    "bought" : true
            },
            {
                    "name" : "my_item_three",
                    "bought" : true
            }
    ]
}

For example how can I modify the first 2 objects of the array and set their "bought" to false. Or in other words how could I change properties of the first n objects.

I thought I could first do a db.findOne() with this ID than take the items and use a function that will change the values and then set the whole item to this new array that this function returns.

That would probably do the work but is there a faster and nicer method? Is there a way to do it if I know the number of objects I want to change, so it will not be n but 5 for example.

Yes you can use .findOne() or similar logic and then alter the content of the document and .save() it back or similar. This is not a "great" pattern though as other writes could have occurred on the document in the time between when you read it and when you save it. If you need to do this kind of updating to "multiple" array elements at once then you are best to consider a different model, ie collection instead of embedded. — user3561036
– user3561036, Commented Jun 12, 2015 at 11:12

Tariq · Accepted Answer · 2015-06-12 11:30:18Z

3

You can loop over the nth objects you need and set the bought to false by using the '=':

for(var i=0; i < n_times; i++){  
    my_object.items[i].bought = false;
}

answered Jun 12, 2015 at 11:30

Tariq

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1 Comment

chnging Over a year ago

Yes but I have to use findOne() and then update() right? I was looking for a solution with one quary.

chridam · Accepted Answer · 2015-06-12 12:33:18Z

1

If you know the number of objects you want to update beforehand then you could take advantage of the dot notation in your update to access the elements of an array with a loop:

var id = ObjectId("4faaba123412d654fe83hg876");

// Iterate and update using the update query object 
for(var i=0; i<n; i++){ 
    var update = { "$set": {} };
    // set the update query object
    update["$set"]["items."+i.toString()+".bought"] = false;
    db.collection.update({"_id": id}, update, {"upsert": false, "multi": true});
}

-- EDIT --

This can also be done in one atomic update, given that you create the update object prior to the update, something like this

var update = { "$set": {} };
for(var i=0; i<n; i++){ 
    // set the update query object
    update["$set"]["items."+i.toString()+".bought"] = false;    
}

db.collection.update({"_id": id}, update, {"upsert": false, "multi": true});

Check the demo below

var update = { "$set": {} };
for(var i=0; i<5; i++){ 
    // set the update query object
    update["$set"]["items."+i.toString()+".bought"] = false;    
}

pre.innerHTML = JSON.stringify(update);

<pre id="pre"></pre>

edited Jun 12, 2015 at 12:33

answered Jun 12, 2015 at 11:38

chridam

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5 Comments

chnging Over a year ago

Yes I was thinking exactly to use the dot notation and that's why I included the part with exact nuber of items in my question. Thank you.

chnging Over a year ago

But don't I do n updates like that? Wouldn't be easier if I construct an update object before that and just make 1 update?

chridam Over a year ago

@chnging Yes that's quite possible, actually would recommend that approach. Let me update my answer with that approach.

chnging Over a year ago

I implemented it like that var setObject = {}; for (var i = 0; i < n; i++) setObject["items." + i.toString() + ".bought"] = false; And then in the update() I use { $set: setObject }

chridam Over a year ago

@chnging Exactly what I had in mind which I updated in the answer. Cheers!

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