How Can I Map Calculated Properties in JPA and Hibernate Using @Formula?

Mapping Calculated Properties with JPA and Hibernate: A Guide

Many Java beans have properties that are not explicitly represented in database tables. These properties might be calculated values, such as the number of child entities associated with a parent entity.

Traditionally, such properties have been set using HQL or the Criteria API. However, the @Formula annotation in Hibernate provides a more efficient and declarative approach.

Using the @Formula Annotation

The @Formula annotation allows you to specify an SQL fragment that calculates the value of a property:

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

In this example, the finalPrice property is calculated by multiplying the PRICE column by 1.155.

Complex Queries and Derived Tables

@Formula can also handle complex queries involving multiple tables:

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

The firstOrderDate property is set by retrieving the minimum creation date from the Orders table for customers with the same id as the current entity.

Performance and Portability Considerations

It's important to note that @Formula uses SQL to calculate values, which can impact performance if used excessively. Additionally, the SQL syntax used in @Formula annotations may not be portable across different database vendors.

For more information, refer to the following resources:

• [Hibernate Derived Properties - Performance and Portability](https://vladmihalcea.com/hibernate-derived-properties-performance-portability/)
• [Hibernate Core documentation: Section 5.1.22. Column and formula elements](https://docs.jboss.org/hibernate/core/5.6/manual/en-US/html/ch05.html#d0e1830)
• [Hibernate Annotations documentation: Section 2.4.3.1. Formula](https://docs.jboss.org/hibernate/orm/6.1/javadocs/org/hibernate/annotations/Formula.html)

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