"""Celko's "Nested Sets" Tree Structure.
http://www.intelligententerprise.com/001020/celko.jhtml
"""
from sqlalchemy import (create_engine,Column,Integer,String,select,case
func)
from sqlalchemy.orm import sessionmaker,MapperExtension,aliased
from sqlalchemy.ext.declarative import declarative_base
engine = create_engine('sqlite://', echo=True)
Base = declarative_base()
class NestedSetExtension(MapperExtension):
def before_insert(self, 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.
# [ticket:1172] needs to be implemented for deletion to work as well.
class Employee(Base):
__tablename__ = 'personnel'
__mapper_args__ = {
'extension':NestedSetExtension(),
'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)
Base.metadata.create_all(engine)
session = sessionmaker(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 his/her 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 his/her 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)
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