db={"orders":[{"_id":1,"item":"almonds","price":12,"quantity":2},{"_id":2,"item":"pecans","price":20,"quantity":1},{"_id":3}],"inventory":[{"_id":1,"sku":"almonds","description":"product 1","instock":120},{"_id":2,"sku":"bread","description":"product 2","instock":80},{"_id":3,"sku":"cashews","description":"product 3","instock":60},{"_id":4,"sku":"pecans","description":"product 4","instock":70},{"_id":5,"sku":null,"description":"Incomplete"}]}

db.orders.aggregate([{"$lookup":{"from":"inventory","localField":"item","foreignField":"sku","as":"inventory_docs"}},{$project:{_id:1,summary:{cusName:"hi",pprice:"$price",/** Include the "price" field*/proName:{$arrayElemAt:["$inventory_docs.sku",0]},instock:{$arrayElemAt:["$inventory_docs.instock",0]}}}}])