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
– Sammaye, Commented May 8, 2014 at 10:10
Then the accepted answer won't actually work, it will get the latest 5 depending upon incrementing ID — Sammaye
– Sammaye, Commented May 8, 2014 at 10:23
This is the closest I got: stackoverflow.com/questions/13967569/… at the end of the day you might find it better to do this application side — Sammaye
– Sammaye, Commented May 8, 2014 at 10:25
Ok I have found something: stackoverflow.com/questions/12786686/… 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
– Sammaye, Commented May 8, 2014 at 10:34

Sebastian · Accepted Answer · 2014-05-08 10:07:52Z

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 8, 2014 at 10:07

Sebastian

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3 Comments

dyk Over a year ago

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?

Sebastian Over a year ago

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.

Sammaye Over a year ago

MongoDB will load everything into memory, the second $match is after a $unwind, as such the entire document will be loaded into memory

Artem Petrosian · Accepted Answer · 2014-05-08 10:11:30Z

0

You can use $elemMatch operator:

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

edited May 8, 2014 at 10:11

answered May 8, 2014 at 9:47

Artem Petrosian

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4 Comments

dyk Over a year ago

(edited) Actually it gives only the first element with type='admin' of the array

dyk Over a year ago

Is there a way to return more that 1 element of the array?

Sammaye Over a year ago

@dyk not yet, the aggregation framework is for that specific purpose

Artem Petrosian Over a year ago

@dyk yep, I agree that for this purpose you should use aggregation framework

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