Solr Grouping - sorting groups based on the sum of the scores of the documents within each group

[George Stathis <gs...@gmail.com>]

Currently, solr grouping (http://wiki.apache.org/solr/FieldCollapsing)
sorts groups "by the score of the top document within each group". E.g.

[...]
"groups":[{
    "groupValue":"81cb63020d0339adb019a924b2a9e0c2",
    "doclist":{"numFound":9,"start":0,"maxScore":4.729042,"docs":[
        {
          "id":"7481df771afe39fab368ce19dfeeb528",
          [...],
          "score":4.729042},
        {
          "id":"c879e95b5f16343dad8b1248133727c2",
          [...],
          "score":4.6635237},
        {
          "id":"485b9aec90fd3ef381f013c51ab6a4df",
          [...],
          "score":4.347174}]
    }},
[...]

Is there an out-of-the-box way to instead sort groups by the sum of the
scores of the documents within each group? E.g.

[...]
"groups":[{
    "groupValue":"81cb63020d0339adb019a924b2a9e0c2",
    "doclist":{"numFound":9,"start":0,*"scoreSum":13.739738*,"docs":[
        {
          "id":"7481df771afe39fab368ce19dfeeb528",
          [...],
          "score":4.729042},
        {
          "id":"c879e95b5f16343dad8b1248133727c2",
          [...],
          "score":4.6635237},
        {
          "id":"485b9aec90fd3ef381f013c51ab6a4df",
          [...],
          "score":4.347174}]
    }},
[...]

With the release of sorting by Function Query (
https://issues.apache.org/jira/browse/SOLR-1297), it seems that there
should be a way to use the sum() function (
http://wiki.apache.org/solr/FunctionQuery). But it's not quite close enough
since the "score" field is not part of the documents.

I feel like I'm close but I'm missing some obvious piece. I'm using Solr
3.5.

Thank you in advance for your time.