# SQLAlchemy 0.9 Documentation

Search:
Release: 0.9.3 | Release Date: February 19, 2014

# Source code for examples.graphs.directed_graph

    """a directed graph example."""

    from sqlalchemy import MetaData, Table, Column, Integer, ForeignKey, \
        create_engine
    from sqlalchemy.orm import mapper, relationship, sessionmaker
    from sqlalchemy.ext.declarative import declarative_base

    Base = declarative_base()

    class Node(Base):
        __tablename__ = 'node'

        node_id = Column(Integer, primary_key=True)

        def __init__(self, id):
            self.node_id = id

        def add_neighbors(self, *nodes):
            for node in nodes:
                Edge(self, node)
            return self

        def higher_neighbors(self):
            return [x.higher_node for x in self.lower_edges]

        def lower_neighbors(self):
            return [x.lower_node for x in self.higher_edges]

    class Edge(Base):
        __tablename__ = 'edge'

        lower_id = Column(Integer,
                            ForeignKey('node.node_id'),
                            primary_key=True)

        higher_id = Column(Integer,
                            ForeignKey('node.node_id'),
                            primary_key=True)

        lower_node = relationship(Node,
                                    primaryjoin=lower_id==Node.node_id,
                                    backref='lower_edges')
        higher_node = relationship(Node,
                                    primaryjoin=higher_id==Node.node_id,
                                    backref='higher_edges')

        # here we have lower.node_id <= higher.node_id
        def __init__(self, n1, n2):
            if n1.node_id < n2.node_id:
                self.lower_node = n1
                self.higher_node = n2
            else:
                self.lower_node = n2
                self.higher_node = n1

    engine = create_engine('sqlite://', echo=True)
    Base.metadata.create_all(engine)

    session = sessionmaker(engine)()

    # create a directed graph like this:
    #       n1 -> n2 -> n5
    #                -> n7
    #          -> n3 -> n6

    n1 = Node(1)
    n2 = Node(2)
    n3 = Node(3)
    n4 = Node(4)
    n5 = Node(5)
    n6 = Node(6)
    n7 = Node(7)

    n2.add_neighbors(n5, n1)
    n3.add_neighbors(n6)
    n7.add_neighbors(n2)
    n1.add_neighbors(n3)

    session.add_all([n1, n2, n3, n4, n5, n6, n7])
    session.commit()

    assert [x.node_id for x in n3.higher_neighbors()] == [6]
    assert [x.node_id for x in n3.lower_neighbors()] == [1]
    assert [x.node_id for x in n2.lower_neighbors()] == [1]
    assert [x.node_id for x in n2.higher_neighbors()] == [5,7]