Java.Hibernate.Medium.What does the @Formula annotation do?

✅ Short Answer

The @Formula annotation lets you map a read-only calculated property in an entity by defining a SQL expression → Hibernate includes it in the SELECT statement when loading the entity.

🔎 Detailed Explanation

🔹 @Formula allows you to expose virtual properties — values not stored as fields in the entity’s table but computed dynamically using SQL.

🔹 The SQL expression you provide in @Formula is inlined into the entity’s SELECT query → it can use:
✅ Columns from the same table.
✅ SQL functions.
✅ Even subqueries.

🔹 Simple Example

@Entity
public class Order {
    @Id
    private Long id;

    private double price;
    private double tax;

    @Formula("price + tax") // computed in SQL SELECT
    private double total;
}

What happens?
When Hibernate loads an Order, it generates:

select id, price, tax, (price + tax) as total from order ...

✅ The total field in Java is populated automatically with the result of the formula.

🔹 Advanced Example: Subquery
Compute a customer’s order count:

@Entity
public class Customer {
    @Id
    private Long id;

    @Formula("(select count(o.id) from orders o where o.customer_id = id)")
    private int orderCount;
}

✅ This gives a read-only field with the number of orders per customer, without a join.

🔹 Important notes
✅ The @Formula value is read-only → you can’t persist changes to it.
✅ The SQL expression must be valid for your database dialect.
❌ Since it’s evaluated by the database, portability may suffer if you use DB-specific SQL functions.

📊 When to use @Formula

✅ Calculated fields based on other columns in the same entity.
✅ Aggregations or subqueries (counts, sums) for convenience without additional queries.
✅ Mapping denormalized or derived data when refactoring is impossible.

📌 Key Takeaways

✅ @Formula lets you map calculated read-only properties using SQL expressions evaluated in Hibernate’s SELECT.
✅ Great for dynamic, computed fields → reduces need for post-processing in Java.
✅ Be careful with database portability, since formulas use native SQL syntax.