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Unexpected output from the card reduces

I have the following map and reduce the functions that should give me the maximum date from the group.

Map

function () { if (this.topic_id != 0) { emit(this.topic_id, {date_posted : this.date_posted}); } }

Reduce:

 function (key, values) { var re_date = ISODate('1970-01-01T00:00:00Z'); values.forEach(function (value) { if (re_date == ISODate('1970-01-01T00:00:00Z')) { re_date = value.date_posted; } if (re_date < value.date_posted) { re_date = value.date_posted; } }); return {date_posted: re_date}; }

Currently it gives the following result:

 { "_id" : "1", "value" : ISODate("2010-06-01T13:36:30Z") } { "_id" : "10", "value" : { "date_posted" : ISODate("2010-05-19T21:33:16Z") } } { "_id" : "1000", "value" : { "date_posted" : ISODate("2010-07-04T04:09:43Z") } } { "_id" : "1004", "value" : ISODate("2010-07-05T01:27:53Z") } { "_id" : "1007", "value" : { "date_posted" : ISODate("2010-07-04T23:44:56Z") } } { "_id" : "1009", "value" : ISODate("2010-08-12T19:05:35Z") } { "_id" : "1012", "value" : ISODate("2010-08-14T19:56:35Z") } { "_id" : "1014", "value" : { "date_posted" : ISODate("2010-07-05T07:18:56Z") } } { "_id" : "1022", "value" : ISODate("2010-07-06T21:17:57Z") } { "_id" : "1024", "value" : { "date_posted" : ISODate("2010-07-06T12:37:28Z") } } { "_id" : "1028", "value" : ISODate("2010-07-08T00:24:59Z") } { "_id" : "1029", "value" : ISODate("2010-07-26T12:08:34Z") } { "_id" : "1042", "value" : { "date_posted" : ISODate("2010-10-01T19:22:29Z") } } { "_id" : "1043", "value" : { "date_posted" : ISODate("2010-07-20T01:38:44Z") } } { "_id" : "1048", "value" : { "date_posted" : ISODate("2010-07-08T19:29:14Z") } } { "_id" : "1049", "value" : ISODate("2010-07-08T11:07:30Z") } { "_id" : "105", "value" : ISODate("2010-05-21T01:53:20Z") } { "_id" : "1053", "value" : { "date_posted" : ISODate("2010-07-08T17:53:57Z") } } { "_id" : "1056", "value" : { "date_posted" : ISODate("2010-07-08T22:35:02Z") } } { "_id" : "1060", "value" : { "date_posted" : ISODate("2010-07-09T06:33:57Z") } }

I expected that the value field for each row will always have a date_posted element, but obviously only some of them have this. Am I having an error in my map function?

Sample source data:

 db.posts.find({"topic_id" : {$in : ["105", "1053", "1000", "1004", "1007"]}}, {"date_posted" : 1, "topic_id" : 1}) { "_id" : "1010", "date_posted" : ISODate("2010-07-05T01:27:53Z"), "topic_id" : "1004" } { "_id" : "106", "date_posted" : ISODate("2010-05-21T01:52:59Z"), "topic_id" : "105" } { "_id" : "107", "date_posted" : ISODate("2010-05-21T01:53:20Z"), "topic_id" : "105" } { "_id" : "1001", "date_posted" : ISODate("2010-07-04T04:09:43Z"), "topic_id" : "1000" } { "_id" : "1006", "date_posted" : ISODate("2010-07-04T23:21:30Z"), "topic_id" : "1004" } { "_id" : "1008", "date_posted" : ISODate("2010-07-04T23:44:56Z"), "topic_id" : "1007" } { "_id" : "1054", "date_posted" : ISODate("2010-07-08T17:53:57Z"), "topic_id" : "1053" }
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1 answer

Well, it looks like my logic for getting the maximum date is bolted. I posted another question on the corresponding issue, and the code posted by William Z works better in response to this question:

 function (key, values) { var re_date = ISODate('1970-01-01T00:00:00Z'); values.forEach(function (val) { if ( re_date < val.date_posted ) re_date = val.date_posted; } ); return {date_posted : re_date}; }
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