2. Retrieving Persistent Objects with SQL
Prev | Chapter 12. SQL Queries | Next

## 2. Retrieving Persistent Objects with SQL

When you give a SQL `Query` a candidate class, it will return persistent instances of that class. At a minimum, your SQL must select the class' primary key columns, discriminator column (if mapped), and version column (also if mapped). The JPA runtime uses the values of the primary key columns to construct each result object's identity, and possibly to match it with a persistent object already in the `EntityManager`'s cache. When an object is not already cached, the implementation creates a new object to represent the current result row. It might use the discriminator column value to make sure it constructs an object of the correct subclass. Finally, the query records available version column data for use in optimistic concurrency checking, should you later change the result object and flush it back to the database.

Aside from the primary key, discriminator, and version columns, any columns you select are used to populate the persistent fields of each result object. JPA implementations will compete on how effectively they map your selected data to your persistent instance fields.

Let's make the discussion above concrete with an example. It uses the following simple mapping between a class and the database:

Example 12.2. Retrieving Persistent Objects

    Query query = em.createNativeQuery("SELECT ISBN, TITLE, PRICE, "
        + "VERS FROM MAG WHERE PRICE > 5 AND PRICE < 10", Magazine.class);
    List<Magazine> results = (List<Magazine>) query.getResultList();
    for (Magazine mag : results)
        processMagazine(mag);

The query above works as advertised, but isn't very flexible. Let's update it to take in parameters for the minimum and maximum price, so we can reuse it to find magazines in any price range:

Example 12.3. SQL Query Parameters

    Query query = em.createNativeQuery("SELECT ISBN, TITLE, PRICE, "
        + "VERS FROM MAG WHERE PRICE > ?1 AND PRICE < ?2", Magazine.class);

    query.setParameter(1, 5d);
    query.setParameter(2, 10d);

    List<Magazine> results = (List<Magazine>) query.getResultList();
    for (Magazine mag : results)
        processMagazine(mag);

Like JDBC prepared statements, SQL queries represent parameters with question marks, but are followed by an integer to represent its index.

Prev | Up | Next
Chapter 12. SQL Queries | Home | Chapter 13. Mapping Metadata