array field filtering by index or another field

[Dmitry B.]

When filtering an array, is it possible:

   1. filter by item index (e.g. get all array elements but the first)?
   2. filter by value in another field within the same document that contains the array field?

Consider a document:

{

    _id: 1, 

     first_seen_on: 20160312, 

     seen_on: [20160312, 20160313, 20160324] 

}

I need the seen_on array to become [20160313, 20160324]

This doesn't seem to work (I think $first_seen_on is treated as literal value): 'repeats': 

   {

       '$filter': { 

           'input': '$seen_on', 

           'as': 'date', 

           'cond': {'$ne': ['$date', '$first_seen_on']

       }

   }

[Wan Bachtiar]

I need the seen_on array to become [20160313, 20160324]

Hi Dmitry,

Given your document example, you could filter the value of seen_on field with first_seen_on field like below:

db.collection.aggregate([
    { $project: 
        { filtered : 
            { $filter: 
                { input: "$seen_on", 
                  as: "date", 
                  cond: {$ne:["$$date", "$first_seen_on"]}
                }
            }
        }
    }
]);

Note that you need to refer to date using double dollar $$ sign. See $filter for more info.

Regards,

Wan.

​