Re: [mongodb-user] $addToSet, array consistency, and size indexing (feature request)

[Scott Hernandez]

There are a few features requests somewhat related to that in jira,
like synthetic fields/index and the update system is due for a rewrite
in the next version or two.

https://jira.mongodb.org/browse/SERVER-340
https://jira.mongodb.org/browse/SERVER-458

Feel free to add requests in jira, and collect votes.

On Sat, Sep 15, 2012 at 12:59 PM, Nathan Smith <na...@branchout.com> wrote:
> Hi all,
>
> We use Mongo in production and an issue I keep running into during the
> design phase is maintaining set consistency in an array using $addToSet
> leaves me unable to guarantee an accurate array size index using $inc on a
> separate field.
>
> In other words, I would like functionality like: $incIffAddToSetSucceeded.
>
> How does the Mongo community feel about this use case? Is there anything new
> being developed that might help?
>
> Specific feature request:
> From searching this forum, it seems like being able to declare a field in a
> document that automatically tracks the size of an array would be extremely
> useful. From my understanding of Mongo and BSON internals, this would be not
> difficult to implement.
>
> Best,
> nate
>
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[Thomas Rueckstiess]

In the mean time, you could change your query for the update to only match the document if the element isn't in the array yet. This is the solution suggested here:

https://groups.google.com/forum/?fromgroups=#!topic/mongodb-user/yaKs4Dcx8P8

So basically, you do something like:

db.collection.update({_id: ObjectId("..."), myarray: {$ne: myelement}}, 

                     {$addToSet: {myarray: myelement}, $inc: {count: 1}})

Thomas