Aggregation pipeline with $or query and $sort (2.6)

[bradle...@twinehealth.com]

Greetings,

Since upgrading to 2.6 I am having an issue with an aggregation pipeline that includes an $or in the $match and a $sort in the pipeline.

Example:

db.createCollection('orAndSort');
db.orAndSort.insert({name: "red", value: 1})
db.orAndSort.insert({name: "red", value: 2})
db.orAndSort.insert({name: "red", value: 3})
db.orAndSort.insert({name: "blue", value: 10})
db.orAndSort.insert({name: "blue", value: 20})
db.orAndSort.insert({name: "green", value: 100})
db.orAndSort.aggregate([
    { '$match': { '$or': [ { name: 'red' }, { name: 'blue' }] } },
    { '$sort': { value: 1 }}
]);

Result:

assert: command failed: {
 "errmsg" : "exception: cursor encountered an error",
 "code" : 17285,
 "ok" : 0
} : aggregate failed
Error: command failed: {
 "errmsg" : "exception: cursor encountered an error",
 "code" : 17285,
 "ok" : 0
} : aggregate failed
    at Error (<anonymous>)
    at doassert (src/mongo/shell/assert.js:11:14)
    at Function.assert.commandWorked (src/mongo/shell/assert.js:244:5)
    at DBCollection.aggregate (src/mongo/shell/collection.js:1149:12)
    at (shell):1:14
2014-04-23T17:40:02.818-0400 Error: command failed: {
 "errmsg" : "exception: cursor encountered an error",
 "code" : 17285,
 "ok" : 0
} : aggregate failed at src/mongo/shell/assert.js:13

Removing either the $sort or the $or condition from the $match causes the pipeline to execute successfully. Thank you for any pointers.

[bradle...@twinehealth.com]

Follow-up,

I have discovered that adding a $project before the $sort avoids the error.

db.orAndSort.aggregate([
    { '$match': { '$or': [ { name: 'red' }, { name: 'blue' }] } },

    { '$project': {name: 1, value: 1}},
    { '$sort': { value: 1 }}
]);

In my actual application, this pattern is probably preferable anyway since I have no need to sort the complete documents. I would still be interested in knowing if this is a bug or other known issue. 

[Mathias Stearn]

You discovered a bug. I went ahead and filed a ticket here: https://jira.mongodb.org/browse/SERVER-13715.

One other work around would be to add an index on {value: 1}. If you do that, it would actually be better to put the $project after the sort rather than before it.

[Asya Kamsky]

Another work-around to the bug (in your particular case, since $or is using the same field in both expressions is to change it to:

db.orAndSort.aggregate([     { '$match': { name: {$in:[ 'red' , 'blue'] } } },     { '$sort': { value: 1 }} ]);

{ "_id" : ObjectId("535977d05168d6c47350f6b7"), "name" : "red", "value" : 1 }

{ "_id" : ObjectId("535977d05168d6c47350f6b8"), "name" : "red", "value" : 2 }

{ "_id" : ObjectId("535977d05168d6c47350f6b9"), "name" : "red", "value" : 3 }

{ "_id" : ObjectId("535977d05168d6c47350f6ba"), "name" : "blue", "value" : 10 }

{ "_id" : ObjectId("535977d05168d6c47350f6bb"), "name" : "blue", "value" : 20 }

Asya

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