How to sort sub-documents in the array field?

Question

I\'m using the MongoDB shell to fetch some results, ordered. Here\'s a sampler,

{
\"_id\" : \"32022\",
\"topics\" : [
    {
        \"weight\" : 281.58551703         


        

Answer 1

It works for me:

db.getCollection('mycollection').aggregate(
{$project:{topics:1}},
{$unwind:"$topics"},
{$sort :{"topics.words":1}})

Answer 2

MongoDB doesn't provide a way to do this out of the box but there is a workaround which is to update your documents and use the $sort update operator to sort your array.

db.collection.update_many({}, {"$push": {"topics": {"$each": [], "$sort": {"weight": -1}}}})

You can still use the .aggregate() method like this:

db.collection.aggregate([
    {"$unwind": "$topics"}, 
    {"$sort": {"_id": 1, "topics.weight": -1}}, 
    {"$group": {"_id": "$_id", "topics": {"$push": "$topics"}}}
])

But this is less efficient if all you want is sort your array, and you definitely shouldn't do that.

---

You could always do this client side using the .sort or sorted function.

Answer 3

If you don't want to update but only get documents, you can use the following query

 db.test.aggregate(
 [
   {$unwind : "$topics"},
   {$sort : {"topics.weight":-1}},
   {"$group": {"_id": "$_id", "topics": {"$push": "$topics"}}}
 ]
)