Cascade Types

What Cascading Means

Without cascading, every entity must be saved on its own: save the order, then save each item. Cascading tells Hibernate: "when I do operation X on the parent, do X on these children too".

CascadeType	Parent operation passed to children
`PERSIST`	`persist()` / `save()` of a new parent saves new children
`MERGE`	`merge()` of a detached parent merges the children
`REMOVE`	deleting the parent deletes the children
`REFRESH`	reloading the parent from the DB reloads the children
`DETACH`	detaching the parent detaches the children
`ALL`	all of the above

Cascade from Order to Items

@OneToMany(mappedBy = "order", cascade = CascadeType.ALL, orphanRemoval = true)
private List<OrderItem> items = new ArrayList<>();

Order order = new Order(user);
order.addItem(new OrderItem(phoneCase, 1, new BigDecimal("299")));
order.addItem(new OrderItem(charger, 2, new BigDecimal("749")));
orderRepository.save(order);      // one call saves the order AND both items

Generated SQL

orderRepository.save(order);

Example output

DEBUG org.hibernate.SQL : insert into orders (status,total,user_id) values (?,?,?)
DEBUG org.hibernate.SQL : insert into order_items (order_id,price,product_id,quantity) values (?,?,?,?)
DEBUG org.hibernate.SQL : insert into order_items (order_id,price,product_id,quantity) values (?,?,?,?)

Where Cascading Belongs

Cascade from a parent to children it owns (whose life depends on the parent). Never cascade towards shared, independent entities.

Association	Cascade?	Why
`Order.items`	`ALL` + orphanRemoval	an item cannot exist without its order
`OrderItem.product`	none	deleting an order must not delete the product
`Product.category`	none	many products share one category
`User.wishlist` (ManyToMany)	never `REMOVE`	it would delete products other users wishlisted

Note: CascadeType.REMOVE on a @ManyToOne or @ManyToMany is almost always a bug. Deleting one order item could delete the product, and with it every other order's reference fails.

Practice

Exercise: A Product has a list of ProductImage rows (URLs of photos). Images are uploaded with the product and deleted with it. Write the mapping on Product.

Show answer

@OneToMany(mappedBy = "product", cascade = CascadeType.ALL, orphanRemoval = true)
@OrderColumn(name = "position")          // optional: keep the photo order
private List<ProductImage> images = new ArrayList<>();

public void addImage(ProductImage img) { images.add(img); img.setProduct(this); }
public void removeImage(ProductImage img) { images.remove(img); img.setProduct(null); }