stats.facet with multi-valued field in Solr 4.9

[Nico Kaiser]

Hi!

I am storing aggregated article click statistics for a website in a Lucene 
database. Website "articles" (i.e., pages in this case) can have multiple 
associated financial instruments, which – for statistics reasons – I also copy 
to Lucene. So basically this data is stored (and regularly updated) "by 
articleId and date" as
{
    articleId: 1234,
    date: "2014-07-21",
    clicks: 50000,
    instrumentIds: [ 1, 2, 3, 4 ]
}

Now I need to generate statistics, like "aggregated article click count by 
instrumentId":

/solr/article_stats/select
    ?q=*:*
    &stats=true
    &stats.field=clicks
    &stats.facet=instrumentIds
    &rows=0

This way Solr returned a (large) list of instrumentIds in 
stats.stats_fields.clicks.facets.instrumentIds with the clicks per instrument, 
which was exactly what I want.


After the upgrade to Solr 4.9 (from 3.6) this seems not to be possible anymore:

"Stats can only facet on single-valued fields, not: instrumentIds"


Is there a way to replicate the old behaviour?

Thanks,
Nico

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