CODEWITHSUNDEEP
neo4j
Trancey
Trancey

Reputation: 709

Making a relation in neo4j

I am not sure what I am doing wrong here, so here is how I create nodes

CREATE (urlnode_1:UrlNode {url:'url1', nodenumber:1})
CREATE (urlnode_2:UrlNode {url:'url2', nodenumber:2})

I create relations as follows

CREATE
(urlnode_1)-[:OutLink {anchor_text:['MY']}]->(urlnode_2)

Two nodes are created successfully first, now on running the code to create the relation, I would have liked the relation to exist between the two created nodes but it creates two new nodes say 3 and 4 and shows a relation between them. What am i doing wrong here?

Upvotes: 1

Views: 75

Answers (2)

Christophe Willemsen
Christophe Willemsen

Reputation: 20185

To guide you the best way I can, let's sum up some Neo4j basics concerning node and relationships creation :

A node can have one or more labels, labels are meaned to group the nodes by domain (User, Speaker, Company, etc..see a label as a table name for e.g. ). A node can also have properties.

A relationship can have only ONE type, relationships are organizing the graph. Relationships can also have properties.

To create a node, you can use the CREATE writing clause : 

CREATE (n:Person {firstname: 'John'})

The CREATE statement will not check if other nodes with same label and properties already exists, it will just create a new node

Relationships can also be created with the same clause :

MATCH (n:Person {firstname: 'John'}), (p:Person {firstname: 'Pierre'})
CREATE (n)-[:KNOWS]->(p)

A complete pattern can also be created in one go :

CREATE (n:Person {name:'Chris'})-[:KNOWS]->(p:Person {name:'Oliver'})

REMINDER : CREATE will not check for existing nodes.

--- AND NOW MERGE ---

MERGE will lazily check for existing nodes, see him as a MATCH OR CREATE clause :

MERGE (n:Person {firstname:'Fred'})

If the node with label Person and firstname Fred does not exist, the node will be created, otherwise nothing will happen. This is where come the handy ON MATCH and ON CREATE mentionned by @joslinm .

If you run this query multiple times after the node creation, your graph will not change, if you know the http protocol, you can say that MERGE is an indempotent request.

Be aware that, MERGE will ensure that an entire pattern exist in the database, by creating it if it does not already exist, meaning that if you do MERGE with a complete pattern, the entire pattern will be looked up for existence, not a single node :

Say a node with label Person and name property with value 'John' already exist in the db :

MERGE (n:Person {name:'John'})

will not affect the graph

However :

MERGE (n:Person {name:'John'})-[:KNOWS]->(:Person {name:'Nathalia'})

A new John node will be created, because the entire pattern does not exist.

It is recommended to use MERGE incrementally :

MERGE (n:Person {name:'John'})
MERGE (p:Person {name:'Nathalia'})
MERGE (n)-[:KNOWS]->(p)

If you want to know more about the MERGE clause, I can highly recommend you this wonderful article from Luanne on GraphAware : http://graphaware.com/neo4j/2014/07/31/cypher-merge-explained.html

Chris

Upvotes: 3

joslinm
joslinm

Reputation: 8105

If you create a relationship, a new one will get created every single time. They are not inherently unique. It sounds like you'd rather be merging the relationship; i.e., if they relationship is there, match it, if not, create it.

The merge syntax for it is as follows:

MERGE (a:Node)-[:LIKES]->(b:Node)
ON 
MATCH SET a.msg = 'I matched!'
ON 
CREATE SET a.msg = 'I created!'
RETURN a

You can try it out here: http://console.neo4j.org/

You'll notice that first the msg will be "I created!" then after it matches, it will be "I matched!"

Upvotes: 2

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