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.

example

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.