Issue with Group By / Field Collapsing

[Shamik Bandopadhyay <sh...@gmail.com>]

Hi,

  I'm trying to use group by option to remove duplicates from my search
result. I'm applying Group By option on a field called TopicId. I'm simply
appending this at the end of my query.

group=true&group.field=TopicId

Initially, the result looked great as I was able to see the duplicates
getting removed and only the document with highest score among the
duplicates,is being returned. But then when I started comparing the result
without the group by option, something doesn't look right. For e.g. the
search without a group by option returned results from Source "A",  "B" and
"C". Documents from Source "A" have the TopicId field while it's not
present in "B" or "C". When I add the Group-By option, the documents from
"B" and "C" are completely ignored, though some of them have scores higher
than A.

I'm little confused if this is the intended behavior.  Does group-by mean
that it'll only return results where the group-by field is present ? Do I
need to use additional group-by parameters to address this ?

Any pointers will be highly appreciated.

Thanks,
Shamik