db={"items":[{"id":"5bb619e49593e5d3cbaa0b52","name":"Flowers","weight":"1.5"},{"id":"5bb619e4ebdccb9218aa9dcb","name":"Chair","weight":"8.4"},{"id":"5bb619e4911037797edae511","name":"TV","weight":"20.8"},{"id":"5bb619e4504f248e1be543d3","name":"Skateboard","weight":"5.9"},{"id":"5bb619e40fee29e3aaf09759","name":"Donald Trump statue","weight":"18.4"},{"id":"5bb619e44251009d72e458b9","name":"Molkkÿ game","weight":"17.9"},{"id":"5bb619e439d3e99e2e25848d","name":"Helmet","weight":"22.7"}],"order":[{"id":"5bb61dfd4d64747dd8d7d6cf","date":"Sat Aug 11 2018 02:01:25 GMT+0000 (UTC)","items":[{"item_id":"5bb619e44251009d72e458b9","quantity":4},{"item_id":"5bb619e4504f248e1be543d3","quantity":2},{"item_id":"5bb619e40fee29e3aaf09759","quantity":3}]}]}

db.order.aggregate([{$unwind:"$items"},{$lookup:{from:"items",localField:"items.item_id",foreignField:"id",as:"item"}},{$unwind:"$item"},{$addFields:{"items.weight":"$item.weight"}},{$group:{_id:"$_id",items:{$push:"$items"},date:{$first:"$date"}}}])