Links can contain properties. These are distinct from links themselves (which we refer to as simply “links”) and are used to store metadata about a link. Due to how they’re persisted under the hood, link properties have a few additional constraints: they’re always single and optional.
In practice, link properties are best used with many-to-many relationships
(multi links without any exclusive constraints). For one-to-one,
one-to-many, and many-to-one relationships the same data should be stored in
object properties instead.
Let’s a create a Person.friends link with a strength property
corresponding to the strength of the friendship.
type Person {
required property name -> str { constraint exclusive };
multi link friends -> Person {
property strength -> float64;
}
}type Person {
required property name -> str { constraint exclusive };
multi link friends -> Person {
property strength -> float64;
constraint expression on (
__subject__@strength >= 0
);
}
}To index on a link property, you must declare an abstract link and extend it.
abstract link friendship {
property strength -> float64;
index on (__subject__@strength);
}
type Person {
required property name -> str { constraint exclusive };
multi link friends extending friendship -> Person;
}The @strength property is specified in the shape of the select
subquery. This is only valid in a subquery inside an insert statement.
insert Person {
name := "Bob",
friends := (
select detached Person {
@strength := 3.14
}
filter .name = "Alice"
)
}We are using the detached operator to unbind the
Person reference from the scope of the insert query.
When doing a nested insert, link properties can be directly included in the
inner insert subquery.
insert Person {
name := "Bob",
friends := (
insert Person {
name := "Jane",
@strength := 3.14
}
)
}update Person
filter .name = "Bob"
set {
friends += (
select .friends {
@strength := 3.7
}
filter .name = "Alice"
)
};The example updates the @strength property of Bob’s friends link to
Alice to 3.7.
In the context of multi links the += operator works like an an insert/update operator.
To update one or more links in a multi link, you can select from the current
linked objects, as the example does. Use a detached selection if you
want to insert/update a wider selection of linked objects instead.
edgedb> ....... ....... ....... ....... .......
select Person {
friends: {
name,
@strength
}
};{
default::Person {name: 'Alice', friends: {}},
default::Person {
name: 'Bob',
friends: {
default::Person {name: 'Alice', @strength: 3.7}
}
},
}