Why is Lazy Loading Not Working for @Formula in Hibernate?

What causes lazy loading to fail when using @Formula in Hibernate?

@Entity
public class Product {
    @Id
    @GeneratedValue
    private Long id;

    private BigDecimal price;

    @Formula("(select count(*) from order_items where product_id = id)")
    private int orderCount;
}

In Hibernate, the use of the @Formula annotation can lead to unexpected behavior with lazy loading. Unlike regular entity mappings, values calculated with @Formula are derived from a database query and are fetched eagerly by default. This can disrupt the expected lazy loading mechanism for associated entities.

// DTO example for avoiding @Formula
define a new class that only pulls necessary data
public class ProductCountDTO {
    private Long id;
    private int orderCount;

    public ProductCountDTO(Long id, int orderCount) {
        this.id = id;
        this.orderCount = orderCount;
    }
}

Causes

• The @Formula annotation executes a subquery each time the parent entity is loaded, making its values inherently eager.
• Hibernate treats @Formula fields as calculated columns, fetching them immediately rather than waiting for access requests.
• The setting of fetch modes and Hibernate caching strategies might conflict with @Formula behavior. When a fetch strategy is not properly declared, it defaults to eager fetching.

Solutions

• To implement lazy loading with @Formula, you can switch to fetching only the required data with `@OneToMany` or `@ManyToOne` relationships rather than using @Formula.
• Consider using a DTO (Data Transfer Object) pattern to fetch data explicitly if complex calculations are needed without violating lazy loading principles.
• Review and adjust Hibernate configurations related to caching and fetching strategies to explicitly manage data accessibility.

Mistake: Assuming lazy loading applies to all fields equally.

Solution: Understand that @Formula behaves differently and usually fetches data eagerly.

Mistake: Not configuring fetch strategies properly in parent-child relationships.

Solution: Use annotations like @BatchSize or explicit fetch strategy settings as needed.