Triggers

This article is a part of a series intended to showcase Memgraph's features and bring the user up to speed on developing with Memgraph.

We highly recommend checking out the other articles from this series which are listed in our how-to guides section.

Introduction

Memgraph supports running openCypher statements after a certain event happens during transaction execution, i.e. triggers.

You can create triggers, delete them, and print them out. All the triggers are persisted on the disk, so no information is lost between the runs.

Creating a trigger

To create a new trigger, a query of the following format should be used:

CREATE TRIGGER trigger_name ( ON ( () | --> ) CREATE | UPDATE | DELETE )
( BEFORE | AFTER ) COMMIT
EXECUTE openCypherStatements

You can find detailed explanations for each part of the trigger further down.

An example would be:

CREATE TRIGGER exampleTrigger
ON UPDATE AFTER COMMIT EXECUTE
UNWIND updatedObjects AS updatedObject
WITH CASE
        WHEN updatedObject.vertex IS NOT null THEN updatedObject.vertex
        WHEN updatedObject.edge IS NOT null THEN updatedObject.edge
    END AS object
SET object.updated_at = timestamp();

The query may seem complex, so let's break it down:

• CREATE TRIGGER exampleTrigger: This statement creates the trigger.
• ON UPDATE AFTER COMMIT EXECUTE: This statement specifies what kind of event should activate the trigger. This one will be triggered for every update operation and the query below will be executed after the update event has been committed.
• UNWIND updatedObjects AS updatedObject: If multiple objects were updated, unwind the list and go over each one.
• WITH CASE...: The CASE expression checks what type of object was updated, a node (vertex) or a relationship (edge).
• SET object.updated_at = timestamp();: Add an updated_at property to the object indicating when the action happened.

Trigger name

Each created trigger must have a globally unique name. This implies that you can't have a pair of triggers with the same name, even if they apply to different events.

Event type

Optionally, users can define on which event a trigger should execute its statements. The event type is defined using the following part:

ON ( () | --> ) CREATE | UPDATE | DELETE

There are three main event types:

• CREATE
• UPDATE
• DELETE

For each event type, users can specify whether to execute the trigger statements only on the events that happened on a vertex, or on an edge. Vertices are denoted with (), and edges with -->.

Few examples would be:

• ON CREATE - trigger the statements only if an object (vertex and/or edge) was created during the transaction execution.
• ON () UPDATE - trigger the statements only if a vertex was updated (e.g. property was set on it) during the transaction execution.
• ON --> DELETE - trigger the statements only if an edge was deleted during the transaction execution.

Each event comes with certain information that can be used in the openCypher statements the trigger executes. The information is contained in the form of predefined variables.

If no event type is specified, the trigger executes its statements every time, and all the predefined variables can be used.

Statement execution phase

A trigger can execute its statements at a specified phase, before or after committing the transaction that triggered it. If the BEFORE COMMIT option is used, the trigger will execute its statements as part of that transaction before it's committed. If the AFTER COMMIT option is used, the trigger will execute its statements asynchronously after that transaction is committed.

Execute statements

A trigger can execute any valid openCypher query. No specific constraints are imposed on the queries. The only way trigger queries (i.e. statements) differ from standard queries is that a trigger query may use predefined variables, which are based on the event type specified for the trigger.

Predefined variables

Statements that a trigger executes can contain certain predefined variables which contain information about the event that triggered it.

Based on the event type, the following predefined variables are available:

Event type	Predefined variables
ON CREATE	createdVertices, createdEdges, createdObjects
ON () CREATE	createdVertices
ON --> CREATE	createdEdges
ON UPDATE	setVertexProperties, setEdgeProperties, removedVertexProperties, removedEdgeProperties, setVertexLabels, removedVertexLabels, updatedVertices, updatedEdges, updatedObjects
ON () UPDATE	setVertexProperties, removedVertexProperties, setVertexLabels, removedVertexLabels, updatedVertices
ON --> UPDATE	setEdgeProperties, removedEdgeProperties, updatedEdges
ON DELETE	deletedVertices, deletedEdges, deletedObjects
ON () DELETE	deletedVertices
ON --> DELETE	deletedEdges
no event type specified	All predefined variables can be used

createdVertices

List of all created vertices.

createdEdges

List of all created edges

createdObjects

List of all created objects where each element is a map. If the element contains a created vertex, it will be in the following format

{
  "event_type": "created_vertex",
  "vertex": created_vertex_object
}

If the element contains a created edge, it will be in the following format

{
  "event_type": "created_edge",
  "edge": created_edge_object
}

deletedVertices

List of all deleted vertices.

deletedEdges

List of all deleted edges

deletedObjects

List of all deleted objects where each element is a map. If the element contains a deleted vertex, it will be in the following format

{
  "event_type": "deleted_vertex",
  "vertex": deleted_vertex_object
}

If the element contains a deleted edge, it will be in the following format

{
  "event_type": "deleted_edge",
  "edge": deleted_edge_object
}

General notes about the predefined variables for updates

Setting an element to NULL is counted as a removal. The changes are looked at on the transaction level only. That means if the value under a property on the same object was changed multiple times, only one update will be generated. The same applies for the labels on the vertex.

setVertexProperties

List of all set vertex properties. Each element is in the following format:

{
  "vertex": updated_vertex_object,
  "key": property_that_was_updated,
  "old": old_value_of_that_property,
  "new": new_value_of_that_property
}

setEdgeProperties

List of all set edge properties. Each element is in the following format:

{
  "edge": updated_vertex_object,
  "key": property_that_was_updated,
  "old": old_value_of_that_property,
  "new": new_value_of_that_property
}

removedVertexProperties

List of all removed vertex properties. Each element is in the following format:

{
  "vertex": updated_vertex_object,
  "key": property_that_was_updated,
  "old": old_value_of_that_property
}

removedEdgeProperties

List of all removed edge properties. Each element is in the following format:

{
  "vertex": updated_vertex_object,
  "key": property_that_was_updated,
  "old": old_value_of_that_property
}

setVertexLabels

List of all set vertex labels. Each element is in the following format:

{
  "label": label,
  "vertices": list_of_updated_vertices
}

removedVertexLabels

List of all removed vertex labels. Each element is in the following format:

{
  "label": label,
  "vertices": list_of_updated_vertices
}

updatedVertices

List of updates consisting of set and removed properties, and set and removed labels on vertices.

updatedEdges

List of updates consisting of set and removed properties on edges.

updatedObjects

List of updates consisting of set and removed properties on edges and vertices, and set and removed labels on vertices.

Elements of the predefined variables for update

Each element has a similar format as the previously defined elements.

If the element contains information about a set vertex property, it's in the following format:

{
  "event_type": "set_vertex_property",
  "vertex": updated_vertex_object,
  "key": property_that_was_updated,
  "old": old_value_of_that_property,
  "new": new_value_of_that_property
}

If the element contains information about a removed vertex property, it's in the following format:

{
  "event_type": "removed_vertex_property",
  "vertex": updated_vertex_object,
  "key": property_that_was_updated,
  "old": old_value_of_that_property
}

If the element contains information about a set edge property, it's in the following format:

{
  "event_type": "set_edge_property",
  "edge": updated_edge_object,
  "key": property_that_was_updated,
  "old": old_value_of_that_property,
  "new": new_value_of_that_property
}

If the element contains information about a removed edge property, it's in the following format:

{
  "event_type": "removed_edge_property",
  "edge": updated_edge_object,
  "key": property_that_was_updated,
  "old": old_value_of_that_property
}

If the element contains information about a set vertex label, it's in the following format:

{
  "event_type": "set_vertex_label",
  "vertex": updated_vertex_object,
  "label": label
}

If the element contains information about a removed vertex label, it's in the following format:

{
  "event_type": "removed_vertex_label",
  "vertex": updated_vertex_object,
  "label": label
}

Owner

The user who executes the create query is going to be the owner of the trigger. Authentication and authorization are not supported in Memgraph Community, thus the owner will always be Null, and the privileges are not checked in Memgraph Community. In Memgraph Enterprise the privileges of the owner are used when executing openCypherStatements, in other words, the execution of the statements will fail if the owner doesn't have the required privileges. More information about how the owner affects the trigger can be found in the reference guide.

Dropping a trigger

A trigger can be removed by running the following query:

DROP TRIGGER trigger_name;

Trigger info

Users can get info about all the triggers by using the following query:

SHOW TRIGGERS;

which returns results in the following format:

trigger name	statement	event type	phase	owner
name of the trigger	statement which the trigger executes	event which triggers the statement	phase at which the trigger executes its statement	owner of the trigger or Null