Understanding the Issue with Persisting Multiple Parents in Spring Data JPA

In this article, we will delve into the intricacies of persisting multiple parents with a single child using Spring Data JPA. We’ll explore the issues that arise when trying to save these entities simultaneously and provide a solution to overcome them.

Introduction to One-To-Many Relationships

Before diving into the problem, let’s first understand how one-to-many relationships work in Java Persistence API (JPA). The @OneToMany annotation on an entity indicates that the annotated entity can have multiple instances of another entity. In return, the associated entity must have a reference to the owning entity through the mappedBy attribute.

For instance, consider an Invoice entity with a list of Discount entities:

@Entity
class Invoice {
    @OneToMany(
            orphanRemoval = true,
            cascade = CascadeType.ALL,
            fetch = FetchType.LAZY,
            mappedBy = "invoice")
    List<Discount> discounts;
}

Here, the discounts field is annotated with @OneToMany, which means an Invoice can have multiple Discount entities. The mappedBy attribute specifies that the owning relationship is mapped to a method named getDiscounts() on the associated entity (Coupon).

The Problem: Saving Multiple Parents with a Single Child

Now, let’s examine the problem at hand. We want to save both an Invoice and a Coupon, which are parents of a single Discount entity. When trying to persist these entities simultaneously, we encounter issues related to transactional behavior and entity references.

The Error: Object References an Unsaved Transient Instance

The error message “object references an unsaved transient instance” indicates that the JPA provider (e.g., Hibernate) detects a circular reference between two or more entities. In this case, the Discount entity is being referenced by both the Invoice and Coupon entities, which are not yet persisted.

This error occurs because Spring Data JPA uses transactional behavior to manage persistence operations. When trying to save multiple parents with a single child, JPA attempts to create a single transaction that encompasses all the saves. However, when dealing with circular references like this one, it can’t determine which entities should be part of the same transaction.

Attempting Separate Transactions

As a workaround, you might try creating separate transactions for each parent entity using @Transactional annotations:

@Transactional(TransactionType.REQUIRES_NEW)
public void createInvoice() {
    Invoice invoice = new Invoice();
    Coupon coupon = couponRepo.query();
    Discount discount = new Discount();
    coupon.discounts.add(discount);
    discount.coupon = coupon;
    invoice.discounts.add(discount);
    invoiceRepo.save(invoice);
}

However, this approach fails because the Discount entity is being referenced by both the Invoice and Coupon entities. When creating a new Discount instance, JPA attempts to save it as part of the first transaction (i.e., the Invoice transaction). Then, when trying to save the Coupon, JPA encounters the circular reference error because the Discount entity is already being saved.

Removing Cascade on One of the Parents

A possible solution involves removing the cascade attribute from one of the parents. Specifically, we can remove it from the Invoice entity:

@Entity
class Invoice {
    @OneToMany(
        fetch = FetchType.LAZY,
        mappedBy = "invoice")
    List<Discount> discounts;
}

By removing the cascade attribute, we ensure that the Discount entity is not automatically saved when persisting the Invoice entity. This allows us to save each parent separately without encountering circular reference errors.

Creating a New Child and Saving the Parent with Cascade

To fix this issue, we need to create a new child (Discount) instance, add it to one of the parents (e.g., Coupon), and then save that parent using saveAndFlush. Afterward, we can update the child by associating it with the other parent and saving the child directly:

@Transactional
public void createInvoice() {
    Invoice invoice = new Invoice();
    Coupon coupon = couponRepo.query();
    Discount discount = new Discount();
    
    // Create a new child and add to parent
    coupon.discounts.add(discount);
    discount.coupon = coupon;
    couponRepo.saveAndFlush(coupon);

    // Save invoice without cascade
    invoiceRepo.save(invoice);

    // Update child by associating with the other parent
    discount.invoice = invoice;
    discountRepo.saveAndFlush(discount);
}

This approach ensures that we save each parent separately, avoiding circular reference errors.

Last modified on 2023-09-21