MongoDB - Rewind an $unwind nested array after $lookup using $group
Because the $lookup operator produces an array field, you need to $unwind the new field before the $group pipeline to get the desired result:
db.users.aggregate([ { "$unwind": "$profile" }, { "$unwind": { "path": "$profile.universities", "preserveNullAndEmptyArrays": true } }, { "$lookup": { "from": "universities", "localField": "profile.universities._id", "foreignField": "_id", "as": "universities" } }, { "$unwind": "$universities" }, { "$group": { "_id": "$_id", "emails": { "$first": "$emails" }, "profile": { "$first": "$profile" }, "universities": { "$push": "$universities" } } }, { "$project": { "emails": 1, "profile.name" : 1, "profile.company": 1, "profile.title" : 1, "profile.phone" : 1, "profile.disabled": 1, "profile.universities": "$universities" } }]).pretty()