some problems with mongoDB - how to get last row value exist on group condition -

i have problem mongo.

i need last name of row if exist. mongo document { _id: object... campaign: 1 total: 1 ...: . .. campaignname: "name of campaign"
}, { _id: object... campaign: 2 total:3 ...: . .. campaignname: "name of campaign 2"
}, { _id: object... campaign: 1 total: 1 ...: . .. campaignname: "z name of campaign"
}, { _id: object... campaign: 1 total: 6 ...: . .. } how see "campaignname" missing on tree position of collection

if this:

db.dataweek.aggregate([{     $project: {         "campaignname": 1     } }, {     $group: {         _id: { "$_id", "$campaign" },         campaignname: {             $last: "$campaignname"         },         total: {             $sum: "$total"         }     } }]) 

i result ok if last rows have field campaignname, if not null name field.

my question how last campaignname value exists ( group campaign ) .

and result should like:

campaign: 1 total: 8 .. campaignname: "z name of campaign"

campaign: 2 total: 3 .. campaignname: "name of campaign 2"

....

you can see name of campaign 1 has changed

ps. $first not valid, need last campaignname exists diferrent ''

in nutshell need $sort documents first before grouping expected result last. consider following sample:

{ "a" : 1, "title" : "here" } { "a" : 1 } { "a" : 1, "title" : "there" } { "a" : 2 } { "a" : 2, "title" : "here" } { "a" : 2, "title" : "there" } { "a" : 1 } { "a" : 2 } 

so find last valid "title" when grouping "a" , obtaining count, first sort grouping key , "title". entries not contain title listed first:

db.collection.aggregate([     { "$sort": { "a": 1, "title": 1 } },     { "$group": {         "_id": "$a",         "title": { "$last": "$title" },         "total": { "$sum": 1 }     }} ]) 

of course results sorted on field "t" come after "h" in example. if had result "sort" on such "date" important include in sort order first:

{ "$sort": { "a": 1, "date": 1, "title": 1 } } 

but there no other way ensure $last cannot

"last if value exists"

the $exists clause query portion remove entries not present, change "count" on results. there $ifnull operator evaluates whether field present , alternately returns value, no use without "sort".

also note single field not looking multiple fields or "last" entry other sorted condition, $max operator same thing without having apply sort.