using $regex in mongodb aggregation framework in $group

Question

Consider the following example:

db.article.aggregate(
  { $group : {
      _id : "$author",
      docsPerAuthor : { $sum : 1 },
      viewsPerAuthor : { $sum : "$pageViews" }
  }}
);

This groups by the author field and computes two fields.

I have values for $author = FirstName_LastName. Now instead of grouping by $author, I want to group by all authors who share the same LastName.

I tried $regex to group by all matching strings after the '_'

$author.match(/_[a-zA-Z0-9]+$/)

db.article.aggregate(
  { $group : {
      _id : "$author".match(/_[a-zA-Z0-9]+$/),
      docsPerAuthor : { $sum : 1 },
      viewsPerAuthor : { $sum : "$pageViews" }
  }}
);

also tried the following:

 db.article.aggregate(
  { $group : {
      _id : {$author: {$regex: /_[a-zA-Z0-9]+$/}},
      docsPerAuthor : { $sum : 1 },
      viewsPerAuthor : { $sum : "$pageViews" }
  }}
);

Answer 1

Use mapReduce: it is the general form of aggregation. This is how to proceed in mongo shell: Define the map function

var mapFunction = function() {
  var key = this.author.match(/_[a-zA-Z0-9]+$/)[0];
  var nb_match_bar2 = 0;
  if( this.bar.match(/bar2/g) ){
    nb_match_bar2 = 1;
  }
  var value = {
    docsPerAuthor: 1,
    viewsPerAuthor: Array.sum(this.pageViews)
  };

  emit( key, value );
};

and the reduce function

var reduceFunction = function(key, values) {

  var reducedObject = {
    _id: key,
    docsPerAuthor: 0,
    viewsPerAuthor: 0
  };

  values.forEach( function(value) {
    reducedObject.docsPerAuthor += value.docsPerAuthor;
    reducedObject.viewsPerAuthor += value.viewsPerAuthor;
  }
  );
  return reducedObject;
};

run mapReduce and save the result in map_reduce_result

>db.st.mapReduce(mapFunction, reduceFunction, {out:'map_reduce_result'})

query map_reduce_result to have the result

>db.map_reduce_result.find()

Answer 2

Actually there is no such method which provides this kind of functionality or i could not find the appropriate version which contains it. That will not work with $regexp i think : http://docs.mongodb.org/manual/reference/operator/regex/ it is just for pattern matching.

There is an improvement request in the jira : https://jira.mongodb.org/browse/SERVER-6773

It is in open unresolved state. BUT

in github i found this disscussion: https://github.com/mongodb/mongo/pull/336

And if you check this commit: https://github.com/nleite/mongo/commit/2dd175a5acda86aaad61f5eb9dab83ee19915709

it contains more or less exactly the method you likely to have. I do not really get the point of the state of this improvement: in 2.2.3 it is not working .