ForeignKey (core 0.8.1 API)

```
@Retention(RUNTIME)
@Target(FIELD)
@Column
public @interface ForeignKey
```
Indicates that a field is a foreign key to another table. It is automatically configured as a column, and cannot be used on a collection attribute.

Optional Element Summary

Optional Elements
Modifier and Type	Optional Element	Description
`boolean`	`cascadeDelete`	Specifies if the foreign entity must be deleted when the owning entity is deleted or the foreign key is set to null.
`ForeignKey.OnForeignDeleted`	`onForeignDeleted`
`boolean`	`optional`	Specifies if the link is optional or not.

- Element Detail
  - optional
```
boolean optional
```
    Specifies if the link is optional or not. In other words, an optional foreign key is nullable.
    This is almost the same as onForeignDeleted, but we may want an optional link to be deleted on forein deleted, for example in a tree, the root is nullable, but if a parent is deleted we want the children to be deleted.
    
    Default:
    
    false
- - onForeignDeleted
```
ForeignKey.OnForeignDeleted onForeignDeleted
```
    Default:
    
    net.lecousin.reactive.data.relational.annotations.ForeignKey.OnForeignDeleted.DELETE
- - cascadeDelete
```
boolean cascadeDelete
```
    Specifies if the foreign entity must be deleted when the owning entity is deleted or the foreign key is set to null.
    
    Default:
    
    false