Release: 1.0.0 | Release Date: Not released

SQLAlchemy 1.0 Documentation

Source code for examples.adjacency_list.adjacency_list

from sqlalchemy import Column, ForeignKey, Integer, String, create_engine
from sqlalchemy.orm import Session, relationship, backref,\
                                joinedload_all
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm.collections import attribute_mapped_collection


Base = declarative_base()

class TreeNode(Base):
    __tablename__ = 'tree'
    id = Column(Integer, primary_key=True)
    parent_id = Column(Integer, ForeignKey(id))
    name = Column(String(50), nullable=False)

    children = relationship("TreeNode",

                        # cascade deletions
                        cascade="all, delete-orphan",

                        # many to one + adjacency list - remote_side
                        # is required to reference the 'remote'
                        # column in the join condition.
                        backref=backref("parent", remote_side=id),

                        # children will be represented as a dictionary
                        # on the "name" attribute.
                        collection_class=attribute_mapped_collection('name'),
                    )

    def __init__(self, name, parent=None):
        self.name = name
        self.parent = parent

    def __repr__(self):
        return "TreeNode(name=%r, id=%r, parent_id=%r)" % (
                    self.name,
                    self.id,
                    self.parent_id
                )

    def dump(self, _indent=0):
        return "   " * _indent + repr(self) + \
                    "\n" + \
                    "".join([
                        c.dump(_indent + 1)
                        for c in self.children.values()]
                    )

if __name__ == '__main__':
    engine = create_engine('sqlite://', echo=True)

    def msg(msg, *args):
        msg = msg % args
        print("\n\n\n" + "-" * len(msg.split("\n")[0]))
        print(msg)
        print("-" * len(msg.split("\n")[0]))

    msg("Creating Tree Table:")

    Base.metadata.create_all(engine)

    session = Session(engine)

    node = TreeNode('rootnode')
    TreeNode('node1', parent=node)
    TreeNode('node3', parent=node)

    node2 = TreeNode('node2')
    TreeNode('subnode1', parent=node2)
    node.children['node2'] = node2
    TreeNode('subnode2', parent=node.children['node2'])

    msg("Created new tree structure:\n%s", node.dump())

    msg("flush + commit:")

    session.add(node)
    session.commit()

    msg("Tree After Save:\n %s", node.dump())

    TreeNode('node4', parent=node)
    TreeNode('subnode3', parent=node.children['node4'])
    TreeNode('subnode4', parent=node.children['node4'])
    TreeNode('subsubnode1', parent=node.children['node4'].children['subnode3'])

    # remove node1 from the parent, which will trigger a delete
    # via the delete-orphan cascade.
    del node.children['node1']

    msg("Removed node1.  flush + commit:")
    session.commit()

    msg("Tree after save:\n %s", node.dump())

    msg("Emptying out the session entirely, "
        "selecting tree on root, using eager loading to join four levels deep.")
    session.expunge_all()
    node = session.query(TreeNode).\
                        options(joinedload_all("children", "children",
                                                "children", "children")).\
                        filter(TreeNode.name == "rootnode").\
                        first()

    msg("Full Tree:\n%s", node.dump())

    msg("Marking root node as deleted, flush + commit:")

    session.delete(node)
    session.commit()