$arrayElemAt throwing invalid operator error in 3.2.4

[epice...@gmail.com]

Hi, I'm running mongodb 3.2.4 on the latest version of El Capitan. I'm trying to write an aggregation pipeline that uses the new $arrayElemAt operator in the $projection phase. However, I keep getting an error that states: E QUERY  [thread1] Error: command failed: 
{ "errmsg" : "exception: invalid operator '$arrayElemAt'", "code" : 15999, "ok" : 0 }.

I've even tried copying and pasting the exact example from the docs, but get the same error. So pretty sure it's not a syntax error. 

Relevant documentation: https://docs.mongodb.org/manual/reference/operator/aggregation/arrayElemAt/

[Asya Kamsky]

I'm pretty sure I don't believe that it's not a syntax error unless you cut and paste the exact output of the following:

db.version()

your full pipeline.

I use the $arrayElemAt expression pretty much all the time and I can promise you it works (on my Mac even).

Asya

P.S. heck, it was still in my scrollback, and it's the exact same version of mongod - 3.2.4:

test@local:27017(3.2.4) > db.went.aggregate({$sort:{name:1,dateTimeSeen:1}}, {$group:{_id:"$name", places:{$push:{place:"$placeSeen",time:"$dateTimeSeen"}}}},{$project:{trips:"$places",places:1}},{$unwind:"$places"},{$sort:{name:1,time:1}},{$project:{"places":1, to: {$arrayElemAt:[ {$filter:{input:"$trips",as:"trip",cond:{$and:[{$ne:["$places.place","$$trip.place"]},{$lt:["$places.time","$$trip.time"]}]}}},0]}}}, {$match:{to:{$ne:null}}} )

{ "_id" : "Sue", "places" : { "place" : "B1", "time" : ISODate("2016-03-16T10:21:41Z") }, "to" : { "place" : "A1", "time" : ISODate("2016-03-16T10:24:41Z") } }

{ "_id" : "Sue", "places" : { "place" : "A1", "time" : ISODate("2016-03-16T10:24:41Z") }, "to" : { "place" : "C1", "time" : ISODate("2016-03-16T10:29:41Z") } }

{ "_id" : "Sue", "places" : { "place" : "C1", "time" : ISODate("2016-03-16T10:29:41Z") }, "to" : { "place" : "B1", "time" : ISODate("2016-03-16T10:35:00Z") } }

{ "_id" : "John", "places" : { "place" : "A1", "time" : ISODate("2016-03-16T10:25:41Z") }, "to" : { "place" : "B1", "time" : ISODate("2016-03-16T10:27:41Z") } }

{ "_id" : "John", "places" : { "place" : "B1", "time" : ISODate("2016-03-16T10:27:41Z") }, "to" : { "place" : "C1", "time" : ISODate("2016-03-16T10:29:41Z") } }

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Asya Kamsky
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