= When searching over multiple databases, how do I convert an MSet entry to the sub-database and docid in it? =

== C/C++ ==

Document ids in the combined database are interleaved, so if {{{docid_combined}}} is the docid from the MSet, and `db` is the `Xapian::Database` object then the following C++ code implements this:

{{{
#!C++
shard_number = (docid_combined - 1) % db.size(); // First shard is 0.
docid_shard = (docid_combined - 1) / db.size() + 1;
}}}

This makes use of the `Database::size()` method which is available in Xapian 1.4.12 and later.  If you need to support older versions you'll need to keep track of the number of shards yourself.  That can be fiddly in some cases, because `Database::add_database()` "flattens" the added database, so doesn't necessarily increase the number of shards by one - if could not increase it (if the added database contains no shards) or increase it by more than one (if the added database contains multiple shards).

== Other languages ==

If you want to implement this in another language you should be able to adapt the C/C++ code above.  Note that
`%` means modulus in the above and `/` an integer division (truncating any fractional part towards zero).

== Omega 1.4.7 and later ==

If you need to do this in an Omega template then 1.4.7 added `$subdb{DOCID}` and `$subid{DOCID}` to give you the name of the sub-database containing ''DOCID'' and the corresponding docid in that sub-database.  You can use `$subdb` and `$subid` if you just want to use the current document in the hitlist.  

== Omega 1.4.6 and older ==

If you need to support older versions of Omega, this is equivalent to `$subdb` without an argument list:

{{{
$slice{$split{/,$dbname},$mod{$sub{$id,1},$length{$split{/,$dbname}}}}
}}}

And this to `$subid` without an argument list:

{{{
$add{$div{$sub{$id,1},$length{$split{/,$dbname}}},1}
}}}

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