Define Hibernate Mapping FileThe <class> elements are used to define specific mappings from a Java classes to the database tables. The Java class name is specified using the name attribute of the class element and the database table name is specified using the table attribute.
The One-To-One mapping represents a single-valued association where an instance of one entity is associated with an instance of another entity. In this type of association one instance of source entity can be mapped atmost one instance of target entity.
Mapping identifier properties. The @Id annotation lets you define which property is the identifier of your entity. This property can be set by the application itself or be generated by Hibernate (preferred). You can define the identifier generation strategy thanks to the @GeneratedValue annotation.
hibernate mappings are one of the key features of hibernate . they establish the relationship between two database tables as attributes in your model. that allows you to easily navigate the associations in your model and criteria queries.
JPA doesn't offer any support for derived property so you'll have to use a provider specific extension. As you mentioned, @Formula
is perfect for this when using Hibernate. You can use an SQL fragment:
@Formula("PRICE*1.155")
private float finalPrice;
Or even complex queries on other tables:
@Formula("(select min(o.creation_date) from Orders o where o.customer_id = id)")
private Date firstOrderDate;
Where id
is the id
of the current entity.
The following blog post is worth the read: Hibernate Derived Properties - Performance and Portability.
Without more details, I can't give a more precise answer but the above link should be helpful.
You have three options:
@Transient
method@PostLoad
entity listener@Formula
annotationWhile Hibernate allows you to use @Formula, with JPA, you can use the @PostLoad callback to populate a transient property with the result of some calculation:
@Column(name = "price")
private Double price;
@Column(name = "tax_percentage")
private Double taxes;
@Transient
private Double priceWithTaxes;
@PostLoad
private void onLoad() {
this.priceWithTaxes = price * taxes;
}
So, you can use the Hibernate @Formula
like this:
@Formula("""
round(
(interestRate::numeric / 100) *
cents *
date_part('month', age(now(), createdOn)
)
/ 12)
/ 100::numeric
""")
private double interestDollars;
Take a look at Blaze-Persistence Entity Views which works on top of JPA and provides first class DTO support. You can project anything to attributes within Entity Views and it will even reuse existing join nodes for associations if possible.
Here is an example mapping
@EntityView(Order.class)
interface OrderSummary {
Integer getId();
@Mapping("SUM(orderPositions.price * orderPositions.amount * orderPositions.tax)")
BigDecimal getOrderAmount();
@Mapping("COUNT(orderPositions)")
Long getItemCount();
}
Fetching this will generate a JPQL/HQL query similar to this
SELECT
o.id,
SUM(p.price * p.amount * p.tax),
COUNT(p.id)
FROM
Order o
LEFT JOIN
o.orderPositions p
GROUP BY
o.id
Here is a blog post about custom subquery providers which might be interesting to you as well: https://blazebit.com/blog/2017/entity-view-mapping-subqueries.html
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