# References and `on_delete`

Any field typed `X.id` is a reference to a record of `X`. The database
gets a foreign key and an index for it. Its block must say what happens
when the record it refers to is deleted; there is no default.

```jadpo
entity User {
    id: Uuid
}

entity Project {
    id: Uuid
}

entity Todo {
    id: Uuid
    owner_id: User.id {
        on_delete: OnDelete.restricted  // a user with todos cannot be deleted
    }
    project_id: Project.id? {
        on_delete: OnDelete.cleared  // the todo stays; it leaves the project
    }
}
```

Here a user who still has todos can't be deleted, and a todo whose project
is deleted stays, outside any project.

| `on_delete:` | When the referenced record is deleted | Allowed on |
|---|---|---|
| `OnDelete.restricted` | The delete fails with a conflict while any record refers to it | any reference |
| `OnDelete.cascade` | Records that refer to it are deleted in the same transaction | any reference |
| `OnDelete.cleared` | The reference is set to `none` | an optional reference (`Project.id?`) |

A cycle of required references is an error, because neither record could
be created first.