How to sort inner array and match elements in mongoose

Question

So I am trying to do a semi-complicated query in mongoose. It is as follows:

Event.findOne({ users: { $elemMatch: { user: someUserId, going: 1 }, sort: {createTime: -1} } }

Basically what I would like to do is find a single Event that has a user in its user array that matches an Id and is attending. I have multiple user records for each user in the user array so I want to find the most recent one, to do this I would like to sort the user array by createTime. This is where the error is coming in and it just returns undefined . It works just fine when I don't include the sort function, is there any way to include that?

Here is what my Event object looks like:

{ 
  _id: 1,
  endTime: 1429060173865,
  createTime: 1429051773902,
  startTime: 1429052973865,
  description: 'some desc',
  creator: 2,
  users:
   [ { user: 1,
       status: 1,
       going: 1,
       createTime: 1429051773749,
       _id: 552d997d8e923847306e2c21 },
     { user: 1,
       status: 1,
       going: 1,
       createTime: 1429051773922,
       _id: 552d997d8e923847306e2c25 },
     { user: 1,
       status: 9,
       going: 0,
       createTime: 1429051773942,
       _id: 552d997d8e923847306e2c26 } ],
  destroyed: 0 }

Is there any way to make this query entirely in mongoose?

Answer 1

As part of find, MongoDB can't sort an array field of a document. You could define a virtual of Mongoose to return the array in sorted order. You could also maintain the array in sorted order, as shown below:

> db.test.drop()
// note: not in order when inserted
> db.test.insert({ "_id" : 0, "users" : [
    { "user" : 1, "going" : 1, "created" : 22 },
    { "user" : 2, "going" : 1, "created" : 775 },
    { "user" : 1, "going" : 1, "created" : 6432 }
] })
// insert user to array and sort as part of update
> db.test.update({ "_id" : 0 },
    { "$push" : { 
        "users" : {
            "$each" : [{ "user" : 2, "going" : 1, "created" : 5532 }],
            "$sort" : { "created" : -1 }
        }
    } })
> > db.test.findOne()
{
    "_id" : 0,
    "users" : [
        { "user" : 1, "going" : 1, "created" : 6432 },
        { "user" : 2, "going" : 1, "created" : 5532 },
        { "user" : 2, "going" : 1, "created" : 775 },
        { "user" : 1, "going" : 1, "created" : 22 }
    ]
}

That way, when you perform your find query, the arrays in the matching documents will already be in the desired order.

Answer 2

Your query is not correctly written. In order to sort you should write it in third argument of find:

Event.findOne(
     { users: 
           { $elemMatch: { user: someUserId, going: 1 }}
     },
     null,
     { 
         sort: {createTime: -1} 
     },
     function(err, event) {
         //your event here
     });