db.a.find()
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{ "_id" : ObjectId("58ebbf82016ce019673b111c"), "a" : [ { "_id" : 1 }, { "_id" : 2 } ] }
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{ "_id" : ObjectId("58ebbf8d016ce019673b111d"), "a" : [ { "_id" : 5 }, { "_id" : 12 } ] }
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> db.b.find()
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{ "_id" : 1 }
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{ "_id" : 2 }
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{ "_id" : 3 }
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{ "_id" : 5 }
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{ "_id" : 12 }
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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"}})
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{ "_id" : ObjectId("58ebbf82016ce019673b111c"), "a" : [ { "_id" : 1 }, { "_id" : 2 } ], "details" : [ ] }
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{ "_id" : ObjectId("58ebbf8d016ce019673b111d"), "a" : [ { "_id" : 5 }, { "_id" : 12 } ], "details" : [ ] }
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"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"}})
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{ "_id" : ObjectId("58ebbf82016ce019673b111c"), "a" : [ 1, 2 ], "details" : [ { "_id" : 1 }, { "_id" : 2 } ] }
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{ "_id" : ObjectId("58ebbf8d016ce019673b111d"), "a" : [ 5, 12 ], "details" : [ { "_id" : 5 }, { "_id" : 12 } ] }
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Bug
Major - P3
SERVER-42306 $lookup with an array of sub objects should allow for an option to allow each sub object to find join partners