DEFINE EVENT
statement
Events can be triggered after any change or modification to the data in a record. Each trigger is able to see the $before
and $after
value of the record, enabling advanced custom logic with each trigger.
Requirements
- You must be authenticated as a root or namespace user before you can use the
DEFINE EVENT
statement. - You must select your namespace and database before you can use the
DEFINE EVENT
statement.
Statement syntax
SurrealQL SyntaxDEFINE EVENT [ OVERWRITE | IF NOT EXISTS ] @name ON [ TABLE ] @table [ WHEN @expression ] THEN @expression [ COMMENT @string ]
Example usage
Below is an example showing how to create an event which upon updating a user's email address will create an entry recording the change on an event
table.
-- Create a new event whenever a user changes their email address
-- One-statement event
DEFINE EVENT email ON TABLE user
WHEN $before.email != $after.email
THEN (
CREATE event SET user = $value.id, time = time::now(), value = $after.email, action = 'email_changed'
);
-- Create a relation between a customer and a product whenever a purchase is made
-- Notice the subtle difference when we use multiple statements inside an event:
-- we have to use {curly brackets} instead of (parenthesis)
DEFINE EVENT purchase ON TABLE purchase
WHEN $before == NONE
THEN {
LET $from = (SELECT * FROM customer WHERE id == $after.customer);
LET $to = (SELECT * FROM product WHERE id == $after.product);
RELATE $from->purchases->$to CONTENT {
quantity: $after.quantity,
total: $after.total,
status: 'Pending',
};
};
Specific events
You can trigger events based on specific events. You can use the variable $event to detect what type of event is triggered on the table.
-- CREATE event is triggered when a new record is inserted into the table.
-- Here we are updating the status of the post to PUBLISHED
-- when a new record is inserted into the publish_post table.
DEFINE EVENT publish_post ON TABLE publish_post
WHEN $event = "CREATE"
THEN (
UPDATE post SET status = "PUBLISHED" WHERE id = $after.post_id
);
-- UPDATE event
-- Here we are creating a notification when a user is updated.
DEFINE EVENT user_updated ON TABLE user
WHEN $event = "UPDATE"
THEN (
CREATE notification SET message = "User updated", user_id = $after.id, created_at = time::now()
);
-- DELETE event is triggered when a record is deleted from the table.
-- Here we are creating a notification when a user is deleted.
DEFINE EVENT user_deleted ON TABLE user
WHEN $event = "DELETE"
THEN (
CREATE notification SET message = "User deleted", user_id = $before.id, created_at = time::now()
);
-- You can combine multiple events based on your use cases.
-- Here we are creating a log when a user is created, updated or deleted.
DEFINE EVENT user_event ON TABLE user
WHEN $event = "CREATE" OR $event = "UPDATE" OR $event = "DELETE"
THEN (
CREATE log SET
table = "user",
event = $event,
happened_at = time::now()
);
Using IF NOT EXISTS
clause
Available since: v1.3.0
The IF NOT EXISTS
clause can be used to define an event only if it does not already exist. You should use the IF NOT EXISTS
clause when defining an event in SurrealDB if you want to ensure that the event is only created if it does not already exist. If the event already exists, the DEFINE EVENT
statement will return an error.
It's particularly useful when you want to safely attempt to define a event without manually checking its existence first.
On the other hand, you should not use the IF NOT EXISTS
clause when you want to ensure that the event definition is updated regardless of whether it already exists. In such cases, you might prefer using the OVERWRITE
clause, which allows you to define a event and overwrite an existing one if it already exists, ensuring that the latest version of the event definition is always in use
-- Create a EVENT if it does not already exist
DEFINE EVENT IF NOT EXISTS example ON example THEN {};
Using OVERWRITE
clause
Available since: v2.0.0
The OVERWRITE
clause can be used to define an event and overwrite an existing one if it already exists. You should use the OVERWRITE
clause when you want to modify an existing event definition. If the event already exists, the DEFINE EVENT
statement will overwrite the existing event definition with the new one.
-- Create an EVENT and overwrite if it already exists
DEFINE EVENT OVERWRITE example ON example THEN {};
Events and permissions
Queries inside the event always execute without any permission checks, even when triggered by changes made by the currently authenticated user. This can be very useful to perform additional checks and changes that involve tables/records that are inaccessible for the user.
Consider a CREATE query sent by a record user that has CREATE access to the comment
table only:
CREATE comment SET
post = post:tomatosoup,
content = "So delicious!",
author = $auth.id
;
By having the following event defined, SurrealDB will perform the additional checks and changes:
DEFINE EVENT on_comment_created ON TABLE comment
WHEN $event = "CREATE"
THEN {
-- Check if the post allows for adding comments.
-- User record doesn't have access to the `post` table.
IF $after.post.disable_comments {
THROW "Can't create a comment - Comments are disabled for this post";
};
-- Set the `approved` field on the new comment - automatically approve
-- comments made by the author of the post.
-- For security reasons, record users don't have any permissions for the `approved` field.
UPDATE $after.id SET
approved = $after.post.author == $after.author;
};