examples.nested_sets.nested_sets
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2023-12-01
"""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.mapped_table right_most_sibling = connection.scalar( select(personnel.c.rgt).where( personnel.c.emp == instance.parent.emp ) ) connection.execute( personnel.update(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))