Hi all,
(All examples below are using Lucene 2.2; if things have changed in
later versions please adjust accordingly, though a quick check of the
classes involved shows no major changes in trunk)
We have an interesting situation where we are effectively indexing two
'entities' in our system, which share a one-to-many relationship
(imagine 'User' and 'Delivery Address' for demonstration purposes). At
the moment, we index one Lucene Document per 'many' end, duplicating the
'one' end data, like so:
userid: 1
userfirstname: fred
addresscountry: au
addressphone: 1234
userid: 1
userfirstname: fred
addresscountry: nz
addressphone: 5678
userid: 2
userfirstname: mary
addresscountry: au
addressphone: 5678
(note: 2 Documents indexed for user 1). This is somewhat annoying for
us, because when we search in Lucene the results we want back
(conceptually) are at the 'user' level, so we have to collapse the
results by distinct user id, etc. etc (let alone that it blows out the
size of our index enormously). So why do we do it? It would make more
sense to use multiple fields:
userid: 1
userfirstname: fred
addresscountry: au
addressphone: 1234
addresscountry: nz
addressphone: 5678
userid: 2
userfirstname: mary
addresscountry: au
addressphone: 5678
But imagine the search "+addresscountry:au +addressphone:5678". We'd
like this to match ONLY Mary, but of course it matches Fred also because
he matches both those terms (just for different addresses).
There are two aspects to the approach we've (more or less) got working
but I'd like to run them past the group and see if they're worth trying
to get them into Lucene proper (if so, I'll create a JIRA issue for them)
1) Use a modified SpanNearQuery. If we assume that country + phone will
always be one token, we can rely on the fact that the positions of 'au'
and '5678' in Fred's document will be different.
SpanQuery q1 = new SpanTermQuery(new Term("addresscountry", "au"));
SpanQuery q2 = new SpanTermQuery(new Term("addressphone", "5678"));
SpanQuery snq = new SpanNearQuery(new SpanQuery[]{q1, q2}, 0, false);
the slop of 0 means that we'll only return those where the two terms are
in the same position in their respective fields. This works brilliantly,
BUT requires a change to SpanNearQuery's constructor (which checks that
all the clauses are against the same field). Are people amenable to
perhaps adding another constructor to SNQ which doesn't do the check, or
subclassing it to do the same (give it a protected non-checking
constructor for the subclass to call)?
2) It gets slightly more complicated in the case of variable-length
terms. For example, imagine if we had an 'address' field ('123 Smith
St') which will result in (1 to n) tokens; slop 0 in a SpanNearQuery
won't work here, of course. One thing we've toyed with is the idea of
using getPositionIncrementGap -- if we knew that 'address' would be, at
most, 20 tokens, we might use a position increment gap of 100, and make
the slop factor 50; this works fine for the simple case (yay!), but with
a great many addresses-per-user starts to get more complicated, as the
gap counts from the last term (so the position sequence for a single
value field might be 0, 100, 200, but for the address field it might be
0, 1, 2, 3, 103, 104, 105, 106, 206, 207... so it's going to get out of
sync). The simplest option here seems to be changing (or supplementing)
public int getPositionIncrementGap(String fieldname)
to
public int getPositionIncrementGap(String fieldname, int currentPos)
so that we can override that to round up to the nearest 100 (or
whatever) based on currentPos. The default implementation could just
delegate to getPositionIncrementGap().
What do people think? Is this ugly, or worth pursuing? Does anyone have
any other, better ideas? I was curious as to whether Hibernate Search
deals with this problem, in terms of many-to-one relationships. However,
it's actually not clear from the documentation whether it actually DOES
or not, so if anyone has insight into that that would be great.
Thanks in advance,
Paul
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