# Unique rules

`unique: true` in a field's block means no two records may hold the same
value in that field. A rule over several fields goes in the entity's
`unique: { }` section, with a name and its fields as a list:

```jadpo
type ProjectName = Text {
    max_length: 80
}

entity User {
    id: Uuid
    email: Email {
        unique: true  // one user per address
    }
}

entity Project {
    id: Uuid
    owner_id: User.id {
        on_delete: OnDelete.cascade
    }
    name: ProjectName

    unique: {
        name_per_owner: [owner_id, name]
    }
}
```

Here one user may own only one project of a given name. A rule's fields
must exist, be different fields, and be required, so every engine agrees
on what a duplicate is. A write that breaks the rule refers to it by its
name when it maps the conflict to a failure
([Conflicts](reads-and-writes.md#conflicts)).

On a tenant's records, a rule holds within each tenant: its key holds the
tenancy reference too
([A unique rule holds within its tenant](tenancy.md#a-unique-rule-holds-within-its-tenant)).