Futures

Futures are values which are only computed when the data is selected and returned to the client. Futures can be stored inside records, to enable dynamic values which are always calculated when queried.

Simple futures

Any value or expression can be used inside a future. This value will be dynamically computed on every access to the record.

CREATE person SET accessed_date = <future> { time::now() };

-- Only used inside a DEFINE FIELD statement
DEFINE FIELD accessed_date ON person COMPUTED time::now();

Futures inside schema definitions

A future can be added to a schema definition as well.

DEFINE FIELD accessed_at ON TABLE user VALUE <future> { time::now() };

CREATE user:one;
SELECT * FROM ONLY user:one;
-- Sleep for one second
SLEEP 1s;
-- `accessed_at` is a different value now
SELECT * FROM ONLY user:one;

DEFINE FIELD accessed_at ON TABLE user COMPUTED time::now();

CREATE user:one;
SELECT * FROM ONLY user:one;
-- Sleep for one second
SLEEP 1s;
-- `accessed_at` is a different value now
SELECT * FROM ONLY user:one;

This differs from a VALUE clause which is only calculated when it is modified (created or updated), but is not recalculated during a SELECT query which does not modify a record.

DEFINE FIELD updated ON TABLE user VALUE time::now();

CREATE user:one;
SELECT * FROM ONLY user:one;
-- Sleep for one second
SLEEP 1s;
-- `updated` is still the same
SELECT * FROM ONLY user:one;

Futures depending on statements

If the value of a future is the result of a statement, it must be wrapped in parentheses.

DEFINE FIELD random_movie
    ON app_screen
    VALUE <future> { (SELECT * FROM ONLY movie ORDER BY RAND() LIMIT 1) };

-- No need for parentheses
DEFINE FIELD random_movie
    ON app_screen
    COMPUTED SELECT * FROM ONLY movie ORDER BY RAND() LIMIT 1;

If your statement is wrapped in parentheses, you need to access the fields using the $parent variable.

DEFINE FIELD OVERWRITE followers
    ON user
    VALUE <future> { (SELECT VALUE count FROM ONLY follower_count WHERE user = $parent.id LIMIT 1) ?? 0 };

Avoiding infinite recursion

When defining a future on a field, be sure to avoid any statements that would cause infinite recursion. In the following example, the random_friend field is defined by a statement that uses a SELECT statement on all the fields of the same person table, one of which will also use the same future to compute its value.

CREATE |person:10| SET name = "Person " + <string>id.id() RETURN NONE;

DEFINE FIELD random_friend
    ON person
    VALUE <future> { (SELECT * FROM ONLY person ORDER BY RAND() LIMIT 1) };

CREATE person;

'Reached excessive computation depth due to functions, subqueries, or futures'

A SELECT query that does not access the field defined by a future will avoid the infinite recursion.

CREATE |person:10| SET name = "Person " + <string>id.id() RETURN NONE;

DEFINE FIELD random_friend
    ON person
    VALUE <future> { (SELECT VALUE name FROM ONLY person ORDER BY RAND() LIMIT 1) };

CREATE person;

[
	{
		id: person:4o973bouhd6xrj8l2x69,
		random_friend: 'Person imoy71qbhnsgjtczybiq'
	}
]

Next steps

You've now seen how to create dynamically computed properties on records, using either simple values, and values which depend on local and remote record fields. Take a look at the next chapter to get into SurrealDB's geospatial types.