db={"players":[{"_id":ObjectId("5fba17c1c4566e57fafdcd7e"),"username":"moshe","items":[{"_id":ObjectId("5fbb5ac178045a985690b5fd"),"equipped":false,"itemId":"5fbb5ab778045a985690b5fc"},{"_id":ObjectId("5fbb5ac178045a985690b5fe"),"equipped":false,"itemId":"5fbb5ab778045a985690b5fd"}]}],"items":[{"_id":ObjectId("5fbb5ab778045a985690b5fc"),"name":"Axe","damage":4,"defense":6},{"_id":ObjectId("5fbb5ab778045a985690b5fd"),"name":"Axe1","damage":3,"defense":4}]}

db.players.aggregate([{$unwind:"$items"},{$lookup:{from:"items",let:{itemId:{$toObjectId:"$items.itemId"},items:"$items"},pipeline:[{$match:{$expr:{$eq:["$_id","$$itemId"]}}},{$replaceRoot:{newRoot:{$mergeObjects:["$$items","$$ROOT"]}}}],as:"items"}},{$group:{_id:"$_id",username:{$first:"$username"},items:{$push:{$first:"$items"}}}}])