MongoDB allowDiskUse not working..

Question

Experts.

I'm new to MongoDB, but know enough to get my self in trouble.. case in point:

db.test.aggregate(
[
    {$group: {_id: {email: "$email", gender: "$gender"}, cnt: {$sum: 1}}}, 
    {$group: {_id: "$_id.email", cnt: {$sum: 1}}}, 
    {$match: {cnt: 2}}
], 
    {allowDiskUse : true}
)

and no matter what variations I try, I keep getting the same error ("Pass allowDiskUse:true to opt in"):

Error("Printing Stack Trace")@:0 ()@src/mongo/shell/utils.js:37 ([object Array],[object Object])@src/mongo/shell/collection.js:866 @(shell):7

uncaught exception: aggregate failed: { "errmsg" : "exception: Received error in response from mongo1.mscnet.com:27017: { $err: \"Exceeded memory limit for $group, but didn't allow external sort. Pass allowDiskUse:true to opt in.\", code: 16945 }", "code" : 16945, "ok" : 0 }

FYI: I'm using Mongo 2.6

Answer 1

Please use aggregate query in run command,it will allow to use allowDiskUse tag.

db.runCommand(
   { aggregate: "test",
     pipeline: [
                {$group: {_id: {email: "$email", gender: "$gender"}, cnt: {$sum: 1}}}, 
                {$group: {_id: "$_id.email", cnt: {$sum: 1}}}, 
                {$match: {cnt: 2}}
               ],
     allowDiskUse: true
   }
)

Answer 2

Try writing like this,

db.stocks.aggregate([
                     { $project : { cusip: 1, date: 1, price: 1, _id: 0 } },
                     { $sort : { cusip : 1, date: 1 } }
                    ],
                     {
                       allowDiskUse: true
                     }
                    ); 

And One more thing,
Your error about maximum document size is inherent to Mongo. Mongo can never return a document (or array thereof) larger than 16 megabytes. I can't tell you why because you didn't mention anything about data size, but it probably means that the document you're building as an end result is too large. Try decreasing the $limit parameter, maybe? Start by setting it to 1, run a test, then increase it and look at how big the result gets when you do that