db={"orders":[{"_id":1,"products":[{productID:1},{productID:2}]},{"_id":2,"products":[{productID:3},{productID:4}]}],"products":[{"_id":1,"description":"product 1","pictureID":1},{"_id":2,"description":"product 2","pictureID":2},{"_id":3,"description":"product 3","pictureID":3},{"_id":4,"description":"product 4","pictureID":4},],"pictures":[{"_id":1,"bloburl":"product1.jpg"},{"_id":2,"bloburl":"product2.jpg"},{"_id":3,"bloburl":"product3.jpg"},{"_id":4,"bloburl":"product4.jpg"},]}

db.products.aggregate([{"$lookup":{"from":"pictures","localField":"pictureID","foreignField":"_id","as":"productPicture"}}])