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db.users.aggregate([{$unwind:"$certifications.certs"},{$lookup:{from:"certifications",localField:"certifications.certs.itemID",foreignField:"itemID",as:"certification"}},{$unwind:"$certification"},{$group:{_id:"$_id",certs:{$push:{$mergeObjects:["$certifications.certs",{country:"$certification.country",description:"$certification.name.key1"}]}},count:{$sum:1}}}])