db={items:[{_id:"7890",name:"item1"},{_id:"6543",name:"item2"}],orders:[{_id:"123456",name:"John Doe",items:[{item:"7890",count:4},{item:"6543",count:4}]}]}

db.orders.aggregate([{"$lookup":{"from":"items","localField":"items.item","foreignField":"_id","as":"items"}}])