MongoDB Lookup Returns an empty array, only when there is no data
The reason is $unwind. Unwind default behaviour of preserveNullAndEmptyArrays is false. Which means in short is if the array is empty or null, remove the document when deconstructing. If you dont need to remove, you can make it as true. So when your array doesn't have any element, the document is removed. Thats you you are getting empty array