Release: 2.1.0b1 | Release Date: January 21, 2026

# SQLAlchemy 2.1 Documentation

# Source code for examples.nested_sets.nested_sets

    """Celko's "Nested Sets" Tree Structure.

    https://www.intelligententerprise.com/001020/celko.jhtml

    """

    from sqlalchemy import case
    from sqlalchemy import Column
    from sqlalchemy import create_engine
    from sqlalchemy import event
    from sqlalchemy import func
    from sqlalchemy import Integer
    from sqlalchemy import select
    from sqlalchemy import String
    from sqlalchemy.ext.declarative import declarative_base
    from sqlalchemy.orm import aliased
    from sqlalchemy.orm import Session

    Base = declarative_base()

    class Employee(Base):
        __tablename__ = "personnel"
        __mapper_args__ = {
            "batch": False  # allows extension to fire for each
            # instance before going to the next.
        }

        parent = None

        emp = Column(String, primary_key=True)

        left = Column("lft", Integer, nullable=False)
        right = Column("rgt", Integer, nullable=False)

        def __repr__(self):
            return "Employee(%s, %d, %d)" % (self.emp, self.left, self.right)

    @event.listens_for(Employee, "before_insert")
    def before_insert(mapper, connection, instance):
        if not instance.parent:
            instance.left = 1
            instance.right = 2
        else:
            personnel = mapper.persist_selectable
            right_most_sibling = connection.scalar(
                select(personnel.c.rgt).where(
                    personnel.c.emp == instance.parent.emp
                )
            )

            connection.execute(
                personnel.update()
                .where(personnel.c.rgt >= right_most_sibling)
                .values(
                    lft=case(
                        (
                            personnel.c.lft > right_most_sibling,
                            personnel.c.lft + 2,
                        ),
                        else_=personnel.c.lft,
                    ),
                    rgt=case(
                        (
                            personnel.c.rgt >= right_most_sibling,
                            personnel.c.rgt + 2,
                        ),
                        else_=personnel.c.rgt,
                    ),
                )
            )
            instance.left = right_most_sibling
            instance.right = right_most_sibling + 1

        # before_update() would be needed to support moving of nodes
        # after_delete() would be needed to support removal of nodes.

    engine = create_engine("sqlite://", echo=True)

    Base.metadata.create_all(engine)

    session = Session(bind=engine)

    albert = Employee(emp="Albert")
    bert = Employee(emp="Bert")
    chuck = Employee(emp="Chuck")
    donna = Employee(emp="Donna")
    eddie = Employee(emp="Eddie")
    fred = Employee(emp="Fred")

    bert.parent = albert
    chuck.parent = albert
    donna.parent = chuck
    eddie.parent = chuck
    fred.parent = chuck

    # the order of "add" is important here.  elements must be added in
    # the order in which they should be INSERTed.
    session.add_all([albert, bert, chuck, donna, eddie, fred])
    session.commit()

    print(session.query(Employee).all())

    # 1. Find an employee and all their supervisors, no matter how deep the tree.
    ealias = aliased(Employee)
    print(
        session.query(Employee)
        .filter(ealias.left.between(Employee.left, Employee.right))
        .filter(ealias.emp == "Eddie")
        .all()
    )

    # 2. Find the employee and all their subordinates.
    # (This query has a nice symmetry with the first query.)
    print(
        session.query(Employee)
        .filter(Employee.left.between(ealias.left, ealias.right))
        .filter(ealias.emp == "Chuck")
        .all()
    )

    # 3. Find the level of each node, so you can print the tree
    # as an indented listing.
    for indentation, employee in (
        session.query(func.count(Employee.emp).label("indentation") - 1, ealias)
        .filter(ealias.left.between(Employee.left, Employee.right))
        .group_by(ealias.emp)
        .order_by(ealias.left)
    ):
        print("    " * indentation + str(employee))