[hibernate-dev] HSearch Projection queries & multiple fields with same name

[Marko Lukša]

Hi,

we've found the following problem regarding projection queries when 
dealing with documents containing multiple fields with the same name.

Let's say we add field "foo" with two different values to the same document:

luceneOptions.addFieldToDocument("foo", "aaa", document);
luceneOptions.addFieldToDocument("foo", "bbb", document);

If we now do a projection query on field "foo", one would expect the 
resultset to contain exactly two results ({"aaa"}  and {"bbb"}), but 
HSearch returns only a single result (the property value of the result 
is either "aaa" or "bbb", because Document.getFieldable("foo"), which is 
called in o.h.search.engine.impl.DocumentBuilderHelper, returns the 
first field that matches the given name).

DocumentExtractor.extract() returns a single EntityInfo, but in order 
for it to properly handle projections as described in the previous 
paragraph, it should really be modified to return List<EntityInfo>.

This sounds pretty reasonable when the query is projecting only a single 
field. When projecting multiple multi-valued fields, the resultset 
should actually return a cartesian product.

This is one way of doing it.  The other way of doing it is if we 
consider multiple fields with the same name as a single multi-valued 
field. When projecting such fields, the resultset would contain the same 
number of results as there are matching documents, with the projected 
value being a collection of all the values stored in the field.

Actually, in CapeDwarf we need the cartesian product, as this is the way 
Google AppEngine does it.

What do you guys think?

Marko