$lookup does not expand 'localField' as expected when it is a dotted path representing fields within an array

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    • Fully Compatible
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    • v3.4
    • Query 2017-05-29
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      db.a.find()
{ "_id" : ObjectId("58ebbf82016ce019673b111c"), "a" : [ { "_id" : 1 }, { "_id" : 2 } ] }
{ "_id" : ObjectId("58ebbf8d016ce019673b111d"), "a" : [ { "_id" : 5 }, { "_id" : 12 } ] }
> db.b.find()
{ "_id" : 1 }
{ "_id" : 2 }
{ "_id" : 3 }
{ "_id" : 5 }
{ "_id" : 12 }

      I would expect $lookup using "a._id" as localField to work since SERVER-22881 was fixed, however:

      db.a.aggregate({$lookup:{from:"b", localField:"a._id", foreignField:"_id", as:"details"}})
{ "_id" : ObjectId("58ebbf82016ce019673b111c"), "a" : [ { "_id" : 1 }, { "_id" : 2 } ], "details" : [ ] }
{ "_id" : ObjectId("58ebbf8d016ce019673b111d"), "a" : [ { "_id" : 5 }, { "_id" : 12 } ], "details" : [ ] }

      "details" is properly populated only if I unwind "a" and use "a._id" or if I create another field first in $addFields or $project from "$a._id" (which makes it a simple array) then it works as localField:

      db.a.aggregate({$addFields:{a:"$a._id"}},{$lookup:{from:"b", localField:"a", foreignField:"_id", as:"details"}})
{ "_id" : ObjectId("58ebbf82016ce019673b111c"), "a" : [ 1, 2 ], "details" : [ { "_id" : 1 }, { "_id" : 2 } ] }
{ "_id" : ObjectId("58ebbf8d016ce019673b111d"), "a" : [ 5, 12 ], "details" : [ { "_id" : 5 }, { "_id" : 12 } ] }

              Assignee:
              James Wahlin
              Reporter:
              Asya Kamsky
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