Re: [mongodb-user] count operator when retrieving subset of fields

[Scott Hernandez]

There are requests for features like this, yes. Fundamentally the data stored on the server doesn't have the count either, without counting all the array elements. In order to make this faster it is best to keep the count up to date yourself by storing a count next to the array in a field like votersCount. Then every time you add something you can also increase the counter. The one tricky part is that you must check to see if your addToSet succeeded or not to know if your were correct to increase the counter.

http://www.mongodb.org/display/DOCS/Updating#Updating-CheckingtheOutcomeofanUpdateRequest

https://jira.mongodb.org/browse/SERVER-1144

On Thu, Jun 28, 2012 at 7:42 AM, mgobeil <marc....@gmail.com> wrote:

Say I have a document like this and I want a voter count:

post { voters: [1, 2, 7, 6] }

I can't increment a separate counter like voterCount without reading the record before updating it to check for duplicates, so when writing I just want to use $addToSet.

But then, on the read side, if all I want is the count of post.voters there doesn't seem to be any way to get it without loading the (possibly very large) list of voters, only to do a .size() on the array and throw it out.

It seems to me like there should be something like slice, but for count:

db.posts.find({}, {voters:{$slice: 5}}) // first 5 voters

db.posts.find({}, {voters:{$count: 1}}) // return count of voters instead of voters array

 

Is a feature like that in the works, or is there another solution I'm missing? 

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