Release: 1.2.0b2 | Release Date: July 24, 2017

SQLAlchemy 1.2 Documentation

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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(
        # 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.

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

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

    def dump(self, _indent=0):
        return "   " * _indent + repr(self) + \
            "\n" + \
                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("-" * len(msg.split("\n")[0]))

    msg("Creating Tree Table:")


    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:")


    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:")

    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.")
    node = session.query(TreeNode).\
        options(joinedload_all("children", "children",
                               "children", "children")).\
        filter( == "rootnode").\

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

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