I am using neo4j for one of my project, there's a node which only has a single property as name, I want to get that node using ID, it already has a ID but when I use this code
MATCH (s:SKILLS{ID:65110}) return s
It returns nothing, heres my node
If the query is wrong then how do I query it using the number
MATCH (s)
WHERE ID(s) = 65110
RETURN s
The ID function gets you the id of a node or relationship. This is different from any property called id or ID that you create.
START should only be used when accessing legacy indexes. It is disabled in Cypher 2.2 3.2 and up.
Neo4j recommends using WHERE ID(n) = , and furthermore states that it will only require a single lookup (does not scan every node to find the matching ID)
WARNING
The answer below is incorrect. It is listed for reference only. See the updated answer above (quoted). The text below is kept for reference only
You can use WHERE ID(s) = 65110, but this will check the ID of every node in your database.
There is a more efficient way to do this:
START s=NODE(517) MATCH(s) RETURN s
you can say:
(n:User) where id(n) >=20 RETURN n
this will return all nodes of type User with node reference ID more than 20
Related
I have a requirement to match two existing nodes with a specific Id and then create a relationship between these nodes.
Below is my cypher. But when I execute this , I always get no changes done .
MATCH(i:`Mechanical Component`)
where ID(i)=9912
with(i)
match(d:Features{name:"Mechanical Component"})
with(d)
where ID(d)=9934
MERGE (i)-[:FEATURES]->(d)
As Frank Pavageau said in the comments, you made an error in your query by not passing i with d in tha second WITHclause. Here is the corrected query you need:
MATCH(i:Mechanical Component)
where ID(i)=9912
with(i)
match(d:Features{name:"Mechanical Component"})
with(d,i)
where ID(d)=9934
MERGE (i)-[:FEATURES]->(d)
Keep in mind that using the internal id is really not recommended since it's generated and may change (see Should we use the Neo4J internal id?). You should probably use your own unique ID (with constraints) and match your node using this ID.
Will Neo4j auto-generate unique ids for all the nodes created using 'CREATE' query, as 'id' in Mysql? We created a node with
CREATE (n: user{name:"x", age:10}) return n
If we want to update 'name' property, how to do it in Neo4j?
From the documentation though,
Searching for nodes by id can be done with the id() function in a predicate.
Neo4j reuses its internal ids when nodes and relationships are deleted. This means that applications using, and relying on internal Neo4j ids, are brittle or at risk of making mistakes. It is therefor recommended to rather use application generated ids.
It is a good idea to as they say, use Application generated ids that are stored as properties of the node. You need to set the application generated id while creating the node and then use that key on merge statements
MERGE (n:user{key:<myApplicationKey>}) return n
SET n.name="x", age=10
There is an internal id, you can access it the id() function
CREATE (n:User {name="x", age: 10}) RETURN n, id(n) as nodeId
Later on, you can update it with
MATCH (n) WHERE id(n) = 12345 SET n.name = "y"
I need to return nodes along with their IDs, and things like this are to be avoided:
RETURN n, ID(n) as n_ID
I would very much prefer to be able to return the ID as a "computed" property of the node, so that the client sees the node as if it had a property named id.
I've been breaking my head over this for hours now. The closest I found is this comment under an unsatisfying answer to a similar question:
MATCH (n:Person) RETURN { id: ID(n), name: n.name } as user
That method is useless because it requires manually reconstructing the node as a map literal in the RETURN clause. This is impossible if you don't know all the property names, which will be the typical scenario in my application; plus, there might be lots of properties.
Next thing I tried was operator '+'. It throws a Type Mismatch; you cannot combine a Map with a Node this way. I cannot find any documented way to convert a Node to a Map either. The docs mention KEYS(), but no VALUES(), nor a way to "zip" keys and values into a map.
Another thing I thought of trying:
REDUCE(theMap = {id: ID(n)}, k in KEYS(n) | theMap + { what? : what? })
... but I know neither how to do a "variable key" in an array literal, nor how to access a node's property by a variable key.
Is what I am trying to do at all possible in cypher?
You can add the id property right after you CREATE a node (and in the same transaction). For example:
CREATE (n:Person {name: "Fred", address: "123 Main St"})
SET n.id = ID(n)
RETURN n;
This way, your nodes will actually have an id node -- there is no need to compute it later.
Depending on what you are doing with the id, you might want to reconsider returning the Neo4j node id (the internal id) and instead generate and store a UUID property on each node. Internal node ids can be reused and should not be used to reference nodes in an external system. Internal ids represent an offset in the store file and if nodes are deleted these gaps in the store file can be filled in, reclaiming already used ids.
There is a plugin here that can automate UUID creation and a discussion on Github here.
I am using Neo4J v2.1.5 and creating a server plugin.
How to create a unique node i.e. guarantee uniqueness of a property?
Is there a hook where in the plugin lifecycle, constraints and indexes can be created?
Returning a node returns the complete database. How can I return just a node or a pojo list as JSON? Are there any working examples or explanation of Representation available?
I am using Java API and not Cypher.
How to create a unique node i.e. guarantee uniqueness of a property?
You can create a unique constraint on a (label, property) pair which will ensure the uniqueness of that property.
e.g.
CREATE UNIQUE CONSTRAINT ON :Person(name)
Would ensure you can't have two people nodes with the same name. If you want to do that from the Java API you'd do something like this:
try ( Transaction tx = graphdb.beginTx() )
{
graphdb.schema()
.constraintFor( DynamicLabel.label( "Person" ) )
.assertPropertyIsUnique( "name" )
.create();
tx.success();
}
Is there a hook where in the plugin lifecycle, constraints and indexes can be created?
You can do that in a transaction but IIRC you can only create one index/constraint per transaction.
Returning a node returns the complete database. How can I return just a node or a pojo list? Are there
any working examples or explanation of Representation available?
Do you mean from cypher? A simple query which will only return one node would be this:
MATCH (n)
RETURN n
LIMIT 1
In cypher land that will return you a map of the properties that the node has on it. If you want to get something more specific you could try this:
MATCH (n:Person)
RETURN n.name AS personName
LIMIT 1
So then you'd get a String back for that column in the result set.
-- Updating for Java API --
From the Java API you can write your own traversals which will give you back 'Node' and 'Relationship' objects. From those you'd then have to extract any properties that you're interested in.
try ( Transaction tx = graphDatabaseService.beginTx() )
{
ResourceIterable<Node> people = GlobalGraphOperations.at( graphDatabaseService ).getAllNodesWithLabel( DynamicLabel.label( "Person" ) );
for ( Node node : people )
{
String name = (String) node.getProperty( "name" );
}
tx.success();
}
Hi with cypher i can sugesst you few thing,
Q How to create a unique node i.e. guarantee uniqueness of a property?
Ans.
first chosse a property that could be unique for that node , same like Primary key in of your relational database system,
i.e Id
now you merge to create a node,
MERGE (u:User { Id:1 })
set u.Name='Charlie'
RETURN u
if the user with Id will not exist it will create it,
then using set Clause you can set other property or hole obejct as well,
Q Returning a node returns the complete database. How can I return just a node or a pojo list as JSON? Are there any working examples or explanation of Representation available?
Ans. Same way to match if you pass the unique id and try to search it will return you only that particualr node only i.e
match(u:User { Id:1 }) return u
to create such Id , i will suggest you to go with GUID created in programing lunguaze like C#,
but with neo4j 3.x you also used autoincremented propery as well.
So I know when you created nodes neo4j has a UUID for each node. I know you can access a particular node by that UUID by accessing the ID. For example:
START n=node(144)
RETURN n;
How would I get the last node that was created? I know I could show all nodes and then run the same command in anotehr query with the corresponding ID, but is there a way to do this quickly? Can I order nodes by id and limit by 1? Is there a simpler way? Either way I have not figured out how to do so through a simple cypher query.
Every time not guaranteed that a new node always has a larger id than all previously created nodes,
So Better way is to set created_at property which stores current time-stamp while creating node.
You can use timestamp() function to store current time stamp
Then,
Match (n)
Return n
Order by n.created_at desc
Limit 1
Please be aware that Neo4j's internal node id is not a UUID. Nor is it guaranteed that a new node always has a larger id than all previously created nodes. The node id is (multiplied with some constant) the offset of the node's location within a store file. Due to space reclaiming a new node might get a lower id number.
BIG FAT WARNING: Do not take any assumption on node ids.
Depending on your requirements you could organize all nodes into a linked list. There is one "magic" node having a specific label, e.g. References that has always a relationship to the latest created node:
CREATE (entryPoint:Reference {to:'latest'}) // create reference node
When a node from your domain is created, you need to take multiple actions:
remove the latest relationships if existing
create your node
connect your new node to the previously latest node
create the reference link
.
MATCH (entryPoint:Reference {to:'latest'})-[r:latest]->(latestNode)
CREATE (domainNode:Person {name:'Foo'}), // create your domain node
(domainNode)-[:previous]->(latestNode), // build up a linked list based on creation timepoint
(entryPoint)-[:latest]->(domainNode) // connect to reference node
DELETE r //delete old reference link
I finally found the answer. The ID() function will return the neo4j ID for a node:
Match (n)
Return n
Order by ID(n) desc
Limit 1;