db={"orders":[{"_id":1,"item":"almonds","price":12,"quantity":2},{"_id":2,"item":"pecans","price":20,"quantity":1}],"items":[{"_id":1,"item":"almonds",description:"almond clusters","instock":120},{"_id":2,"item":"bread",description:"raisin and nut bread","instock":80},{"_id":3,"item":"pecans",description:"candied pecans","instock":60}]}

db.orders.aggregate([{$lookup:{from:"items",localField:"item",/** field in the orders collection*/foreignField:"item",/** field in the items collection*/as:"fromItems"}},{$replaceRoot:{newRoot:{$mergeObjects:[{$arrayElemAt:["$fromItems",0]},"$$ROOT"]}}},{$project:{_id:1,description:1,item:1}}])