Cascading and orphan removal

Key points

• Cascade means: whatever I do to the parent, do to these children. The options are PERSIST, MERGE, REMOVE, REFRESH, DETACH.
• CascadeType.ALL is right when a child cannot exist alone. An order line without its order is nonsense.
• orphanRemoval = true deletes a child as soon as you remove it from the collection. Cascade REMOVE only reacts to deleting the parent.
• Never cascade REMOVE from a @ManyToOne. Deleting one book would delete its author, and every other book with it.
• Orphan removal deletes row by row. For a bulk clean-up write a delete query instead.

Example

@Entity
public class Order {

    @Id @GeneratedValue private Long id;

    @OneToMany(mappedBy = "order",
               cascade = CascadeType.ALL,   // persist/merge/remove flow down
               orphanRemoval = true)        // dropped from the list = DELETE
    private List<OrderLine> lines = new ArrayList<>();

    public void addLine(OrderLine line) {
        lines.add(line);
        line.setOrder(this);
    }

    public void removeLine(OrderLine line) {
        lines.remove(line);      // orphanRemoval turns this into a DELETE
        line.setOrder(null);
    }
}

Cascade along real ownership only: parent to child, never child to parent.