[SERVER-4899] aggregation: need an expression to give the size of an array in projections - MongoDB Jira

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Some analytics queries can't be done by the aggregation framework only for the lack of an array size operator:

var a1 = db.runCommand({ aggregate:"track", pipeline:[

    {$group: {

        _id: "$urllower",

        visits: {$sum: "$visits"},

        kw: {$addToSet: "$kw"}

},

     {$project: {

         urllower: "$_id",

         visits: 1,

         keywords: {$size: "$kw"}

]});

The request here is for the $size operator at the end of the projection, which would give the size of the keyword array being built up here.

I choose $size for the name because we already use that for the same meaning here:
http://www.mongodb.org/display/DOCS/Advanced+Queries#AdvancedQueries-%24size .

is depended on by

DOCS-1949 Document: aggregation: need an expression to give the size of an array in projections

is related to

SERVER-447 new aggregation framework

related to

SERVER-8582 Extend document expression language in aggregation to support advanced document filtering.

Participants:: Adam, Ajay Sabhaney, Alvin Richards, Asya Kamsky, auto, Bjørn Stachmann, Dan, Daniel Pasette, David Henderson, Jared Rosoff, Matt Dannenberg, Nicklas Overgaard, Ryan Behan