Question

Is it possible to order a MongoDB collection by the size of a nested array? Say for example we have a collection of Question documents and each document has a nested array of answers. I want to be able to sort the collection and pull out the most answered questions? I have been looking around and I am not sure its doable directly from MongoDB and I think it would be quite performance intensive to extract all questions and then sort them in Java.

Answer 1

You cann’t query by size of nested collection, you need to create field with size of collection for such needs(mongo db documentation):

The $size operator matches any array
with the specified number of elements.
The following example would match the
object {a:[“foo”]}, since that array
has just one element:

db.things.find( { a : { $size: 1 } }
);

You cannot use $size to find a range
of sizes (for example: arrays with
more than 1 element). If you need to
query for a range, create an extra
size field that you increment when you
add elements.