Just been looking in to this, and it seems there are actually several
issues here (all in Xapian).

The first is that MSetItem's support for the tuple interface via
xapian.MSET_* wasn't documented apart from its use in an older version
of the simpleindex.py example program.  Despite not being explicitly
deprecated, this feature no longer works in Xapian 1.2.

The only working way to use xapian.MSET_* in 1.2 is via mset.items, and
that's the change I suggested in msg4220.  The reason that didn't
actually fix this is that the member of the tuple corresponding to
xapian.MSET_DOCUMENT isn't set (this is a bug since it is documented to
work, but actually it never has worked!)

Our (Xapian devs) The plan to sort out this mess is to note in the
deprecation document that xapian.MSET_* on MSetItem doesn't work in
1.2.x (essentially to retroactively deprecate it to match its removal -
not ideal, but we think the best option at this point), and that
xapian.MSET_DOCUMENT is no longer useful.  We'll also now mark
xapian.MSet.items and xapian.MSET_* as deprecated (but not remove them
for a release cycle).  The preferred API is to use the properties, which
is nicer and works in both 1.0 and 1.2.

This ticket has more details:

http://trac.xapian.org/ticket/531

I've attached a patch against roundup SVN HEAD which should fix this,
but which I haven't tested.  This should still retain compatibility with
Xapian 1.0 too.