How Can I Map Calculated Properties in JPA and Hibernate Without Using HQL or Criteria API?

Mapping Calculated Properties with JPA and Hibernate

Background:

Suppose you have a Java bean with a property that is not mapped to a database column directly. Instead, you want it calculated by the database using an operation like COUNT(). You seek a way to map this property without resorting to HQL or the Criteria API.

Solution Using Hibernate's @Formula:

JPA does not directly support derived properties. However, Hibernate provides the @Formula annotation as a solution. Using this annotation, you can specify a formula for the property, which can include SQL fragments or complex queries:

@Formula("COUNT(childColumns)")
private int childCount;

Example Formulas:

• Calculating a price with a markup:

@Formula("PRICE*1.155")
private float finalPrice;

• Finding the minimum order date for a customer:

@Formula("(select min(o.creation_date) from Orders o where o.customer_id = id)")
private Date firstOrderDate;

Benefits of @Formula:

• Allows you to map calculated properties without additional HQL or Criteria API queries.
• Can improve performance by avoiding unnecessary database round-trips.
• Supports portability across different database platforms.

Additional Resources:

• [Hibernate Derived Properties - Performance and Portability](https://thoughts-on-java.org/hibernate-derived-properties-performance-and-portability/)
• [Hibernate Core documentation on Formula Elements](https://docs.jboss.org/hibernate/core/4.3/manual/en-US/html/ch05.html#formula-support)
• [Hibernate Annotations documentation on Formula](https://docs.jboss.org/hibernate/annotations/3.5/reference/en/html/entity.html#d0e735)

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