@unique annotation
The @unique annotation is used
to specify unique attributes for entities and relations.
Syntax
<type label> owns <attribute type label> @unique;The @unique annotation does not accept any arguments.
Usage
The @unique annotation can be defined for an ownership to put the unique constraint on it.
The unique constraint means that no two instances (owners) of the owner type can have instances (attributes) of the attribute type with the same value.
That makes every owned attribute unique by value among all instances of the owner type and its subtypes.
For example, the following statement defines that every user should have a unique phone:
define
entity user owns phone @unique;
attribute phone value string;With this constraint, no instance of user can share the same phone value.
Subtyping
The unique constraint would also affect the following schema extension:
define
entity uk-user sub user, owns uk-phone;
entity usa-user sub user, owns usa-phone;
phone @abstract;
attribute uk-phone sub phone;
attribute usa-phone sub phone;With the unique constraint on the phone, there is a protection ensuring that:
-
Any
usercan only have one uniquephone. -
A
uk-usercannot have auk-phonewith the same value as ausa-user'susa-phone.
If there is a need to restrict uniqueness within subtypes (e.g., allowing uk-user and usa-user to have phones with overlapping values, but still unique within their respective regions), the unique constraint should be applied directly to the subtype ownerships instead.
Limitations
Cannot be used with the @key annotation as they both produce the unique constraint.
See Supported value types for the list of supported value types.