OnDelete (Hibernate Javadocs)

```
@Target({METHOD,FIELD,TYPE})
@Retention(RUNTIME)
public @interface OnDelete
```
Specifies an on delete action for a foreign key constraint. The most common usage is @OnDelete(action = CASCADE).
```
 @ManyToOne
 @OnDelete(action = CASCADE)
 Parent parent;
 
```
Note that this results in an on delete cascade clause in the DDL definition of the foreign key. It's completely different to CascadeType.REMOVE.
In fact, @OnDelete may be combined with cascade=REMOVE.
```
 @ManyToOne(cascade = REMOVE)
 @OnDelete(action = CASCADE)
 Parent parent;
 
```
- If @OnDelete(action = CASCADE) is used in conjunction with cascade=REMOVE, then associated entities are fetched from the database, marked deleted in the persistence context, and evicted from the second-level cache.
- If @OnDelete(action = CASCADE) is used on its own, without cascade=REMOVE, then associated entities are not fetched from the database, are not marked deleted in the persistence context, and are not automatically evicted from the second-level cache.
Like database triggers, on delete actions can cause state held in memory to lose synchronization with the database.

- Required Element Summary
  
  Required Elements
  Modifier and Type Required Element Description
  
  OnDeleteAction action
  The action to taken by the database when deletion of a row would cause the constraint to be violated.

- Element Detail
  - action
```
OnDeleteAction action
```
    The action to taken by the database when deletion of a row would cause the constraint to be violated.