Triggers
In a trigger you register Cypher statements that are called when data in Neo4j is changed (created, updated, deleted). You can run them before or after commit.
Enable apoc.trigger.enabled=true in $NEO4J_HOME/conf/apoc.conf first.
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add a trigger statement under a name,
in the statement you can use |
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remove previously added trigger, returns trigger information |
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removes all previously added triggers , returns trigger information |
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update and list all installed triggers |
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it pauses the trigger |
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it resumes the paused trigger |
The transaction data from Neo4j is turned into appropriate data structures to be consumed as parameters to your statement, i.e. $createdNodes.
The parameters available are:
| Statement | Description |
|---|---|
transactionId |
returns the id of the transaction |
commitTime |
return the date of the transaction in milliseconds |
createdNodes |
when a node is created our trigger fires (list of nodes) |
createdRelationships |
when a relationship is created our trigger fires (list of relationships) |
deletedNodes |
when a node is delated our trigger fires (list of nodes) |
deletedRelationships |
when a relationship is delated our trigger fires (list of relationships) |
removedLabels |
when a label is removed our trigger fires (map of label to list of nodes) |
removedNodeProperties |
when a properties of node is removed our trigger fires (map of key to list of map of key,old,node) |
removedRelationshipProperties |
when a properties of relationship is removed our trigger fires (map of key to list of map of key,old,relationship) |
assignedLabels |
when a labes is assigned our trigger fires (map of label to list of nodes) |
assignedNodeProperties |
when node property is assigned our trigger fires (map of key to list of map of key,old,new,node) |
assignedRelationshipProperties |
when relationship property is assigned our trigger fires (map of key to list of map of key,old,new,relationship) |
You can use these helper functions to extract nodes or relationships by label/relationship-type or updated property key.
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function to filter entries by label, to be used within a trigger statement with |
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function to filter propertyEntries by property-key, to be used within a trigger statement with $assignedNode/RelationshipProperties and $removedNode/RelationshipProperties. Returns [{old,[new],key,node,relationship}] |
Triggers Examples
We could add a trigger that when is added a specific property on a node, that property is added to all the nodes connected to this node
Dataset
CREATE (d:Person {name:'Daniel'})
CREATE (l:Person {name:'Mary'})
CREATE (t:Person {name:'Tom'})
CREATE (j:Person {name:'John'})
CREATE (m:Person {name:'Michael'})
CREATE (a:Person {name:'Anne'})
CREATE (l)-[:DAUGHTER_OF]->(d)
CREATE (t)-[:SON_OF]->(d)
CREATE (t)-[:BROTHER]->(j)
CREATE (a)-[:WIFE_OF]->(d)
CREATE (d)-[:SON_OF]->(m)
CREATE (j)-[:SON_OF]->(d)
Now we add the trigger using apoc.trigger.propertiesByKey on the surname property
CALL apoc.trigger.add('setAllConnectedNodes','UNWIND apoc.trigger.propertiesByKey($assignedNodeProperties,"surname") as prop
WITH prop.node as n
MATCH(n)-[]-(a)
SET a.surname = n.surname', {phase:'after'});So when we add the surname property on a node, it’s added to all the nodes connected (in this case one level deep)
MATCH (d:Person {name:'Daniel'})
SET d.surname = 'William'
The surname property is add/change on all related nodes
Dataset
CREATE (k:Actor {name:'Keanu Reeves'})
CREATE (l:Actor {name:'Laurence Fishburne'})
CREATE (c:Actor {name:'Carrie-Anne Moss'})
CREATE (m:Movie {title:'Matrix'})
CREATE (k)-[:ACT_IN]->(m)
CREATE (l)-[:ACT_IN]->(m)
CREATE (c)-[:ACT_IN]->(m)
We add a trigger using apoc.trigger.nodesByLabel that when the label Actor of a node is removed, update all labels Actor with Person
CALL apoc.trigger.add('updateLabels',"UNWIND apoc.trigger.nodesByLabel($removedLabels,'Actor') AS node
MATCH (n:Actor)
REMOVE n:Actor SET n:Person SET node:Person", {phase:'before'})MATCH(k:Actor {name:'Keanu Reeves'})
REMOVE k:Actor
We can add a trigger that connect every new node with label Actor and as name property a specific value
CALL apoc.trigger.add('create-rel-new-node',"UNWIND $createdNodes AS n
MATCH (m:Movie {title:'Matrix'})
WHERE n:Actor AND n.name IN ['Keanu Reeves','Laurence Fishburne','Carrie-Anne Moss']
CREATE (n)-[:ACT_IN]->(m)", {phase:'before'})CREATE (k:Actor {name:'Keanu Reeves'})
CREATE (l:Actor {name:'Laurence Fishburne'})
CREATE (c:Actor {name:'Carrie-Anne Moss'})
CREATE (a:Actor {name:'Tom Hanks'})
CREATE (m:Movie {title:'Matrix'})
We have the possibility to pause a trigger without remove it, if we will need it in the future
When you need again of a trigger paused
For this example, we would like that all the reference node properties are of type STRING
CALL apoc.trigger.add("forceStringType",
"UNWIND apoc.trigger.propertiesByKey($assignedNodeProperties, 'reference') AS prop
CALL apoc.util.validate(apoc.meta.type(prop) <> 'STRING', 'expected string property type, got %s', [apoc.meta.type(prop)]) RETURN null", {phase:'before'})CREATE (a:Node) SET a.reference = 1
Neo.ClientError.Transaction.TransactionHookFailedCALL apoc.trigger.add('timestamp','UNWIND $createdNodes AS n SET n.ts = timestamp()');
CALL apoc.trigger.add('lowercase','UNWIND $createdNodes AS n SET n.id = toLower(n.name)');
CALL apoc.trigger.add('txInfo', 'UNWIND $createdNodes AS n SET n.txId = $transactionId, n.txTime = $commitTime', {phase:'after'});
CALL apoc.trigger.add('count-removed-rels','MATCH (c:Counter) SET c.count = c.count + size([r IN $deletedRelationships WHERE type(r) = "X"])')
CALL apoc.trigger.add('lowercase-by-label','UNWIND apoc.trigger.nodesByLabel($assignedLabels,'Person') AS n SET n.id = toLower(n.name)')
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Description |
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The trigger will be activate right |
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The trigger will be activate right after the |
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The trigger will be activate right |
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The trigger will be activate right |