db={"orders":[{"_id":1,"itemId":"111","price":12,"quantity":2},{"_id":2,"itemId":"222","price":20,"quantity":1}],"items":[{"_id":111,"item":"almonds","description":"almond clusters","instock":120},{"_id":155,"item":"bread","description":"raisin and nut bread","instock":80},{"_id":222,"item":"pecans","description":"candied pecans","instock":60}]}

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