如何在现有的两个节点之间建立关系，我正在使用neo4j。

Question

I'm in starting neo4j and I'm using python3.5 and py2neo.

I had build two graph node with following code. and successfully create.[!

>>> u1 = Node("Person",name='Tom',id=1)
>>> u2 = Node('Person', name='Jerry', id=2)
>>> graph.create(u1,u2)

after that, I going to make a relation between 'Tom' and 'Jerry' Tom's id property is 1, Jerry's id property is 2.

So. I think, I have to point to existing two node using id property. and then I tried to create relation like below.

>>> u1 = Node("Person",id=1)
>>> u2 = Node("Person",id=2)
>>> u1_knows_u2=Relationship(u1, 'KKNOWS', u2)
>>> graph.create(u1_knows_u2)

above successfully performed. But the graph is something strange.

I don't know why unknown graph nodes are created. and why the relation is created between unknown two node.

Answer 1

You can have two nodes with the same label and same properties. The second node you get with u1 = Node("Person",id=1) is not the same one you created before. It's a new node with the same label/property.

When you define two nodes (ie your new u1 and u2 ) and create a relationships between them, the whole pattern will be created.

To get the two nodes and create a relationship between them you would do:

# create Tom and Jerry as before
u1 = Node("Person",name='Tom',id=1)
u2 = Node('Person', name='Jerry', id=2)
graph.create(u1,u2)

# either use u1 and u2 directly
u1_knows_u2 = Relationship(u1, 'KKNOWS', u2)
graph.create(u1_knows_u2)

# or find existing nodes and create a relationship between them
existing_u1 = graph.find_one('Person', property_key='id', property_value=1)
existing_u2 = graph.find_one('Person', property_key='id', property_value=2)

existing_u1_knows_u2 = Relationship(existing_u1, 'KKNOWS', existing_u2)
graph.create(existing_u1_knows_u2)

find_one() assumes that your id properties are unique.

Answer 2

Note also that you can use the Cypher query language with Py2neo:

graph.cypher.execute('''
   MERGE (tom:Person {name: "Tom"})
   MERGE (jerry:Person {name: "Jerry"})
   CREATE UNIQUE (tom)-[:KNOWS]->(jerry)
''')

The MERGE statement in Cypher is similar to "get or create". If a Person node with the given name "Tom" already exists it will be bound to the variable tom , if not the node will be created and then bound to tom . This, combined with adding uniqueness constraints allows for avoiding unwanted duplicate nodes.

Answer 3

检查此查询，

MATCH (a),(b) WHERE id(a) =1 and id(b) = 2 create (a)-[r:KKNOWS]->(b) RETURN a, b