"""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 Session, aliased
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import event

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))

