Hi,

I've a document which contains an array of nested object.
I need to have the global sum of the size of array.
I've written an agrgegation:

db.values.aggregate( 
    [
        {$group : 	{
                        _id     : null , 
                     	count	:   { 
                                        $sum: {$size : '$markers'}
                                    }
                                }
        } 
    ])

My collection contains 2610537 documents, so the query is very long. (Too long for me)

I think it will be quicker to have an intermadiate field with the markers size. But I cannot write the update query:

db.values.update(
    { 'sample' : 1753068}, // Used to reduce documents and test.
    { 
        '$set': {
            'count': { $size : markers}
        } 
    },
    {
        upsert : false
    }
)

This give me this error:
ReferenceError: markers is not defined (shell):5

But 'markers' is my arrays of nested objects. so this field exist.

After reading the doc, $size is not part of update operators...

Is there a way to handle this case? (I know , I can do this on update or insert process)

Thanks

Mickael