MongoDB: sorted arrays

Question

Consider the following MongoDB document:

SomeObject {
    nested_objects_ids : [
        ObjectId( "1..." ), 
        ObjectId( "2..." ),
        ...
        ObjectId( "N..." )
    ]
}

The length of nested_object_ids is not limited. Is there an elegant way to keep the nested_object_ids array sorted after pushing arbitrary values (i.e. ObjectIds)?

Thank you!

What are you sorting by? ObjectId itself? If you want them sorted by insertion order that should already happen. — Russell
– Russell, Commented Mar 17, 2012 at 14:09
+1 for the question because I'd also like to know if there is a good solution. As far as I know, if you want them in a numerical sort order, you'd actually need to write back ($set) the entire array in a sorted form rather than simply using $push : anObjectId — Mick Sear
– Mick Sear, Commented Mar 17, 2012 at 14:45
@Russel, for example, first an object with _id "45..." is inserted, the next object has _id = "42...". It will be inserted to the end of the array, but I'd like to have it right before the object with id="45...". — Zaur Nasibov
– Zaur Nasibov, Commented Mar 17, 2012 at 15:04
I think what I'm getting at it why are you using ObjectIds for sorting? They won't sort into any meaningful order. If you're overriding with a custom value for the _id field such as an incrementing int you lose a lot of the benefits of an ObjectId so I would consider revisiting your design. — Russell
– Russell, Commented Mar 17, 2012 at 18:50

Gates VP · Accepted Answer · 2012-03-17 16:10:19Z

2

Is there an elegant way to keep the nested_object_ids array sorted after pushing arbitrary values?

Unfortunately, there is nothing I would consider "elegant".

The $push command does not work here. Your only option is to pull the entire sub-array into the client and then re-write it with a $set.

Honestly, when it comes to dealing with "arrays of objects", MongoDB has limited functionality. You can update with $push, $pull and you can index on an object field, but that's about it.

It's difficult to update a specific sub-object. And querying doesn't return the sub-object, but instead returns the whole document. You could filter it down to returning nested_object_ids, but you always get the whole set there.

A question for you: why do the nested objects need to be sorted?

answered Mar 17, 2012 at 16:10

Gates VP

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1 Comment

Zaur Nasibov Over a year ago

Thank you very much for commenting. Consider the situation where ObjectIds kept in this array would come from different non-synchronized sources, so that e.g. ObjectId_n < ObjectId_(n - 1). And you'd like to keep those sorted, because there are hundreds of such entries in the array, and it takes time to sort it every time a new ObjectId is added. I was hoping that there is a way to "tell" MongoDB to keep a particular array in e.g. binary tree :) Maybe someday...

Francesca Krihely · Accepted Answer · 2013-08-02 19:15:27Z

2

You can now do this with the $push to sorted array feature: https://jira.mongodb.org/browse/SERVER-8008

answered Aug 2, 2013 at 19:15

Francesca Krihely

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2 Comments

neo post modern Over a year ago

I'd like to add that SERVER-991 documentation provides more information on $sort within $push.

lol Over a year ago

It looks like you can use $orderBy from that feature request... But I am running mongo through meteor (so mongo 2.6) and it doesn't look like I can do this...?

quahada · Accepted Answer · 2013-01-30 19:11:44Z

1

The upcoming 2.4 release of Mongodb will allow for sorted $push.

answered Jan 30, 2013 at 19:11

quahada

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