aggregation count subdocument length

[Raitucarp]

I have documents like this:
{

    _id: 030341,

    title: "this is title',

   content: "this is content",

   comments:[

        {name: 'baba', content: 'very long comments', _id: 'beX23', postedAt: ....},

        {name: 'rere', content: 'very big data', _id: 'beX24', postedAt: ....},
        .........

  ]

}

How to use aggregation to count comments length? I want the result have such this:

{
   title: 'title',

   content: 'the content'
   commentCount: 5
}, .....

[Jeff Lee]

I believe you need to unwind the array:

e.g.

db.posts.aggregate(

{ $unwind: '$comments'},

{ $group: { _id:{ title:'$title', content:'$content'}, thecount:{$sum:1}}}

) ;

I would probably store the count in the top level document though...it will probably be cheaper in the long run.

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[Raitucarp]

Oh Okay, I see, the title and the content, is in _id.. Yeah this is the solution. Thanks jeffl

I would probably store the count in the top level document though...it will probably be cheaper in the long run.

What do you mean? $inc comments_count every I push new comment? Could you give me an insight about that?

[Jeff Lee]

Yup.  Your mileage may vary since you can get the counts using aggregation instead of map-reduce.

Also, just noticed that the query I posted assumes that title/content are unique.  You may have to tweak that if it's not the case.

Regards.

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