## SELECT NEW in JPQL: Power Tool or Potential Pitfall?

JPQL's Power or Peril: Using SELECT NEW to Create Objects

The realm of Java Persistence Query Language (JPQL) offers a powerful feature known as SELECT NEW, enabling the creation of new objects within a select statement. Instead of retrieving instances managed by the persistence context, this construct facilitates the manipulation of transient objects. However, the question remains: is SELECT NEW a tool to be embraced or avoided?

Justified Embraces

According to the EJB 3.0 JPA Specification, SELECT NEW serves specific purposes:

• Returns custom Java instances, including non-mapped classes.
• Type-safe retrieval of specific properties from joined entities.

An example would be a query retrieving customer details without the full customer entity:

SELECT NEW com.acme.example.CustomerDetails(c.id, c.status, o.count)
FROM Customer c JOIN c.orders o
WHERE o.count > 100

When to Avoid

While SELECT NEW offers conveniences, it should not be employed indiscriminately. Its prudent use is limited to scenarios where:

• Avoiding full entity retrieval for improved performance or specific use cases.
• Retrieving properties from entities that are not fully joined or mapped.

Embracing Good Practices

Far from discouraging SELECT NEW usage, it is crucial to embrace it while adhering to good practices:

• Avoid using it as a substitute for properly designed joins.
• Consider the potential performance implications when creating large collections of unmanaged objects.
• Limit its use to cases where it enhances the expressiveness or efficiency of the query.

Conclusion

SELECT NEW is a valuable tool when applied appropriately. By embracing its benefits while adhering to best practices, developers can leverage the power of JPQL while avoiding pitfalls. Whether SELECT NEW should be embraced or avoided is thus a question of judicious application, recognizing its utility in specific contexts without succumbing to excessive reliance.

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