How do I combine two Lucene queries using OR?

Question

I'd like to search my index on two fields called "a" and "b". I am given searches like Freud -- theories of psychology and I'd like to perform the following query:

(a="Freud" AND b="theories of psychology") OR (b="Freud" AND a="theories of psychology")

How do I do this? So far I have Lucene constructing the two halves (firstHalf and secondHalf) using MultiFieldQueryParser, then I've combined them with

BooleanQuery combined = new BooleanQuery();
combined.add(firstHalf, BooleanClause.Occur.SHOULD);
combined.add(secondHalf, BooleanClause.Occur.SHOULD);

But combined allows results to be returned where only "theories" is found and not "psychology", where I definitely want both terms. It seems like Lucene is splitting "theories of psychology" into three words and combining them individually with OR. How do I prevent this?

firstHalf looks like:

Query firstHalf = MultiFieldQueryParser.parse(Version.LUCENE_33,
         new String[]{"Freud", "theories of psychology"},
         new String[]{"a", "b"},
         new BooleanClause.Occur[]{BooleanClause.Occur.MUST, BooleanClause.Occur.MUST},
         analyzer);

where analyzer is just a StandardAnalyzer object.

Answer 1

Figured it out myself, but now the code is significantly longer; if anyone knows a more elegant solution, please post and I'll gladly reward. :) (Although I'll be making this into a method shortly...but here's the full version of what's going on...)

QueryParser parser = new QueryParser(Version.LUCENE_33, "a", analyzer);
parser.setDefaultOperator(QueryParser.AND_OPERATOR);
Query a_0 = parser.parse("Freud");
parser = new QueryParser(Version.LUCENE_33, "b", analyzer);
parser.setDefaultOperator(QueryParser.AND_OPERATOR);
Query b_1 = parser.parse("theories of psychology");

BooleanQuery firstHalf = new BooleanQuery();
firstHalf.add(a_0, BooleanClause.Occur.MUST);
firstHalf.add(b_1, BooleanClause.Occur.MUST);

parser = new QueryParser(Version.LUCENE_33, "b", analyzer);
parser.setDefaultOperator(QueryParser.AND_OPERATOR);
Query b_0 = parser.parse("Freud");
parser = new QueryParser(Version.LUCENE_33, "a", analyzer);
parser.setDefaultOperator(QueryParser.AND_OPERATOR);
Query a_1 = parser.parse("theories of psychology");

BooleanQuery secondHalf = new BooleanQuery();
secondHalf.add(b_0, BooleanClause.Occur.MUST);
secondHalf.add(a_1, BooleanClause.Occur.MUST);

BooleanQuery combined = new BooleanQuery();
combined.add(firstHalf, BooleanClause.Occur.SHOULD);
combined.add(secondHalf, BooleanClause.Occur.SHOULD);

Turns out SHOULD does work the way I need it to here. Hopefully someone finds this helpful and I'm not just talking to myself in public ;)

Answer 2

Standard analyzer will tokenize. So the query theories of psychology is equivalent to theories OR of OR psychology.

If you want to search for the phrase "theories of psychology" use a PhraseQuery, or else note that the default QueryParser will interpret quotes as meaning a phrase (i.e. change your code to be "\"theories of psychology\"").

And yes, there is a sense in which Lucene doesn't use Boolean logic, but it's technical and not really relevant here.