[hibernate-dev] RE: Exposing legacy Lucene documents on searches.

[Hardy Ferentschik]

Hi John,

> By the way could you please explain your 'projection' to me. I don't
> understand the concept.

The idea behind projections is to avoid full database round-trips in uses  
cases where you are only interested in a subset of object properties   
(instead of the full Hibernate managed object). Lucene allows you to store  
the original values of the fields you are indexing in the index itself.  
This functioanlity is exposed in HSearch via the annotation  
@Field(store=Store.YES).

My favourite use case is a online shop. Normally such a site has some sort  
of search functionality. Say that only the title, price and category of an  
item is searchable and beeing displayed on the search result page. If you  
store these properties in the lucene index you can retrieve them at search  
time and just return these 'projected' values without accessing the  
database. When a user then clicks on a single item you would normally  
retrieve the fully managed Item object from the database (probably using  
the hibernate id)

Projections will be returned as object arrays. Have a look at the example  
in the HSearch documentation:
http://www.hibernate.org/hib_docs/search/reference/en/html_single/#d0e1139

Cheers
Hardy