Interface Relationship

  • All Superinterfaces:
    Entity, PropertyContainer

    public interface Relationship
    extends Entity
    A relationship between two nodes in the graph. A relationship has a start node, an end node and a type. You can attach properties to relationships with the API specified in PropertyContainer.

    Relationships are created by invoking the Node.createRelationshipTo() method on a node as follows:

    Relationship rel = node.createRelationshipTo( otherNode, MyRels.REL_TYPE );

    The fact that the relationship API gives meaning to start and end nodes implicitly means that all relationships have a direction. In the example above, rel would be directed from node to otherNode. A relationship's start node and end node and their relation to Direction.OUTGOING and Direction.INCOMING are defined so that the assertions in the following code are true:

     Node a = graphDb.createNode(), b = graphDb.createNode();
     Relationship rel = a.createRelationshipTo( b, MyRels.REL_TYPE );
     // Now we have: (a) --- REL_TYPE ---> (b)
     assert rel.getStartNode().equals( a );
     assert rel.getEndNode().equals( b );
     assert rel.getNodes()[0].equals( a ) && rel.getNodes()[1].equals( b );
    Even though all relationships have a direction they are equally well traversed in both directions so there's no need to create duplicate relationships in the opposite direction (with regard to traversal or performance).

    Furthermore, Neo4j guarantees that a relationship is never "hanging freely," i.e. getStartNode(), getEndNode(), getOtherNode(Node) and getNodes() are guaranteed to always return valid, non-null nodes.

    A relationship's id is unique, but note the following: Neo4j reuses its internal ids when nodes and relationships are deleted, which means it's bad practice to refer to them this way. Instead, use application generated ids.

    • Method Detail

      • getId

        long getId()
        Returns the unique id of this relationship. Ids are garbage collected over time so they are only guaranteed to be unique during a specific time span: if the relationship is deleted, it's likely that a new relationship at some point will get the old id. Note: This makes relationship ids brittle as public APIs.
        Specified by:
        getId in interface Entity
        The id of this relationship
      • delete

        void delete()
        Deletes this relationship. Invoking any methods on this relationship after delete() has returned is invalid and will lead to NotFoundException being thrown.
      • getStartNode

        Node getStartNode()
        Returns the start node of this relationship. For a definition of how start node relates to directions as arguments to the relationship accessors in Node, see the class documentation of Relationship.
        the start node of this relationship
      • getEndNode

        Node getEndNode()
        Returns the end node of this relationship. For a definition of how end node relates to directions as arguments to the relationship accessors in Node, see the class documentation of Relationship.
        the end node of this relationship
      • getOtherNode

        Node getOtherNode(Node node)
        A convenience operation that, given a node that is attached to this relationship, returns the other node. For example if node is a start node, the end node will be returned, and vice versa. This is a very convenient operation when you're manually traversing the graph by invoking one of the getRelationships() operations on a node. For example, to get the node "at the other end" of a relationship, use the following:

        Node endNode = node.getSingleRelationship( MyRels.REL_TYPE ).getOtherNode( node );

        This operation will throw a runtime exception if node is neither this relationship's start node nor its end node.

        node - the start or end node of this relationship
        the other node
        RuntimeException - if the given node is neither the start nor end node of this relationship
      • getNodes

        Node[] getNodes()
        Returns the two nodes that are attached to this relationship. The first element in the array will be the start node, the second element the end node.
        the two nodes that are attached to this relationship
      • getType

        RelationshipType getType()
        Returns the type of this relationship. A relationship's type is an immutable attribute that is specified at Relationship creation. Remember that relationship types are semantically equivalent if their names are equal. This is NOT the same as checking for identity equality using the == operator. If you want to know whether this relationship is of a certain type, use the isType() operation.
        the type of this relationship
      • isType

        boolean isType(RelationshipType type)
        Indicates whether this relationship is of the type type. This is a convenience method that checks for equality using the contract specified by RelationshipType, i.e. by checking for equal names.
        type - the type to check
        true if this relationship is of the type type, false otherwise or if null

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