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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();

// 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();

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16.4. Projection Queries

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