db={"inventory":[{"_id":ObjectId("5e8df3c02e197074f39f61ea"),"tag":ObjectId("5e89a1af96d5d8b30aead768"),"ean":"5707196199178","location":"shelf 1"},{"_id":ObjectId("5e8df211727079cdc24e20e1"),"ean":"5707196199178","location":"shelf 1"}],"tags":[{"_id":ObjectId("5e7d174fc63ce5b0ca80b89a"),"nfc":{"id":"04:5f:ae:f2:c2:66:81"},"barcode":{"code":"29300310","type":"EAN8"}},{"_id":ObjectId("5e89a1af96d5d8b30aead768"),"nfc":{"id":"04:48:af:f2:c2:66:80"},"barcode":{"code":"29300716","type":"EAN8"}},{"_id":ObjectId("5e7d1756c63ce5b0ca80b89c"),"nfc":{"id":"04:02:ae:f2:c2:66:81"},"barcode":{"code":"29300648","type":"EAN8"}}]}

db.tags.aggregate([{$lookup:{from:"inventory",localField:"_id",foreignField:"tag",as:"join"}},{$match:{"join":{$size:0}}},{$project:{join:0}}])