16.4. Projection Queries
In many cases returning the full domain object is unnecessary, and only a small subset of attributes are desired by the application. Projection Queries allow a specific subset of attributes (or attribute paths) to be returned. If a projection query is used then the
Query.list() will not return the whole domain entity (List<Object>), but instead will return a List<Object[]>, with each entry in the array corresponding to a projected attribute.
To define a projection query use the
select(...) method when building the query, as seen in the following example:
Example 16.4. Retrieving title and publication year
// Match all books that have the word "engine" in their title or description
// and return only their title and publication year.
Query query = queryFactory.from(Book.class)
.select(Expression.property("title"), Expression.property("publicationYear"))
.having("title").like("%engine%")
.or().having("description").like("%engine%")
.toBuilder()
.build();
// results.get(0)[0] contains the first matching entry's title
// results.get(0)[1] contains the first matching entry's publication year
List<Object[]> results = query.list();