MongoDB Get partial array of document

Question

Data Structure:

{
  _id: '1',
  'title: 'Blabla',
  comments: [ {id:'3', type:'normal'} , 
              {id: '4', type: 'admin'}, 
              {id: '5', type: 'admin'}, 
              {id: '6', type: 'admin'}
            ]
}

What i am trying to get is an array of last 5 comments of type admin:

{ comments: [{id:4, type:'admin'}, {id: '5', type: 'admin'}, {id: '6', type: 'admin'}] }

What i have tried:

db.collection('posts').find({_id: '1', 'comments.type':'admin'}
                             , {
                               comments:{$slice: -5}, 
                               comments:1, _id:0
                              })

But it returns the hole comments array if at least 1 item with type admin exists.

Are the latest defined by incrementing IDs? Or are they just defined by their position in the array? — Sammaye, May 08 '14 at 10:10
Then the accepted answer won't actually work, it will get the latest 5 depending upon incrementing ID — Sammaye, May 08 '14 at 10:23
This is the closest I got: http://stackoverflow.com/questions/13967569/array-subset-in-aggregation-framework-pipeline at the end of the day you might find it better to do this application side — Sammaye, May 08 '14 at 10:25
Ok I have found something: http://stackoverflow.com/questions/12786686/can-the-mongodb-aggregation-framework-group-return-an-array-of-values the answer by Stennie there shows the use of an undocumented operator `$const` to add a field containing an array index to each element as he groups it back up, you can sort on that and then proceed with Sebastians answer — Sammaye, May 08 '14 at 10:34
the workaround i see here is: have an insert date field on comment array and sort by it — dyk, May 08 '14 at 10:39
Indeed normally comments would be dated, i.e. to get comments by date range — Sammaye, May 08 '14 at 10:43

score 4 · Answer 1 · answered May 08 '14 at 10:07

4

You can try to use the aggregation framework:

db.collection.aggregate([ 
  { $match: {_id: "1"} }, 
  { $unwind: "$comments" }, 
  { $match: { "comments.type": "admin" } },
  { $sort: { "comments.id": -1 } }, 
  { $limit: 5 } 
])

This will result in a list of the last 5 comments for _id="1":

{ "_id" : "1", "title" : "abc", "comments" : { "id" : "7", "type" : "admin" } }
{ "_id" : "1", "title" : "abc", "comments" : { "id" : "6", "type" : "admin" } }
{ "_id" : "1", "title" : "abc", "comments" : { "id" : "5", "type" : "admin" } }
{ "_id" : "1", "title" : "abc", "comments" : { "id" : "4", "type" : "admin" } }
{ "_id" : "1", "title" : "abc", "comments" : { "id" : "3", "type" : "admin" } }

answered May 08 '14 at 10:07

Sebastian

16,813
4
49
56

I see, while it does work, is it a good option for performance? If i get the hole array, and run it thought 'for' cycle in node to match my needs, would it be slower? – dyk May 08 '14 at 10:19
The advantage is that Mongo does not need to send the data to the client to perform the operation. Therefore doing this in the database will be faster. – Sebastian May 08 '14 at 10:25
MongoDB will load everything into memory, the second $match is after a $unwind, as such the entire document will be loaded into memory – Sammaye May 08 '14 at 10:27

Artem Petrosian · Answer 2 · 2014-05-08T10:11:30.840

0

You can use $elemMatch operator:

db.collection('posts').find(
    {
        _id: '1',
    },
    {
        comments: {
            $elemMatch: {
                'type': 'admin'
            }
        }
    }
)

edited May 08 '14 at 10:11

answered May 08 '14 at 09:47

Artem Petrosian

2,964
1
22
30

(edited) Actually it gives only the first element with type='admin' of the array – dyk May 08 '14 at 09:58
Is there a way to return more that 1 element of the array? – dyk May 08 '14 at 10:02
@dyk not yet, the aggregation framework is for that specific purpose – Sammaye May 08 '14 at 10:09
@dyk yep, I agree that for this purpose you should use aggregation framework – Artem Petrosian May 08 '14 at 10:12

MongoDB Get partial array of document

2 Answers2