Re: [mongodb-user] Using concat with aggregation...

[Shane Spencer]

> String(1) + String(1)

11

On Fri, May 24, 2013 at 11:55 AM, Steve Jerman <sje...@gmail.com> wrote:

I am trying to use an aggregation command to extract a monthly summary of data.

This projection works....

'$project'=>{ 

    'update'=> { 

    :year =>{ "$year" => "$fcsDate" },

    :month =>{ "$month" => "$fcsDate" }

    },

    'projectName'=>1,

'fcsDate'=>1,

   

    }},

  {"update":{"month":10,"year":2013},"fcsDate":"2013-10-14T00:00:00Z","_id":"519d06cf0c80e16a3800002f","projectName":"Project 3"}

....

What I really want is:

  {"update":"2013-10","fcsDate":"2013-10-14T00:00:00Z","_id":"519d06cf0c80e16a3800002f","projectName":"Project 3"}

But concat doesn't seem to work with numbers:

   {'$project'=>{ 

    'update'=> { 

    :year =>{ "$year" => "$fcsDate" },

    :month =>{ "$month" => "$fcsDate" }

    },

    'projectName'=>1,

'fcsDate'=>1,

   

    }},

   {'$project'=>{ 

    'month'=> { "$concat" => [ "$update.year","-","$update.month"]},

    'projectName'=>1,

'fcsDate'=>1,

   

    }},

/Library/Ruby/Gems/1.8/gems/mongo-1.8.6/lib/mongo/db.rb:548:in `command': Database command 'aggregate' failed: (errmsg: 'exception: $concat only supports strings, not NumberInt32'; code: '16702'; ok: '0.0'). (Mongo::OperationFailure)

from /Library/Ruby/Gems/1.8/gems/mongo-1.8.6/lib/mongo/collection.rb:667:in `aggregate'

from tree.rb:25

is there a function to convert a number to a string?

Steve

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[Ross Lawley]

Hi,

$concat only works on strings and there is no expression in the aggregation framework to cast values into different types.

However, $substr forces coercion into a string, so you could use that:

db.test.drop()

db.test.save({"date": ISODate()})

db.test.aggregate(

  {$project: 

    {"year": {$substr: ["$date", 0, 4]}, 

     "month": {$substr: ["$date", 5, 2]}}}, 

  {$project: 

    {"year-date": {$concat: ["$year", "-", "$month"]}}}

)

Ross