"nested sets" example. needs work.

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diff --git a/examples/nested_sets/nested_sets.py b/examples/nested_sets/nested_sets.py
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+"""Celko's "Nested Sets" Tree Structure.
+
+http://www.intelligententerprise.com/001020/celko.jhtml
+
+"""
+
+from sqlalchemy import *
+from sqlalchemy.orm import *
+from sqlalchemy.orm import attributes
+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.
+# (the between() operator in SQLAlchemy has a bug here, [ticket:1171])
+ealias = aliased(Employee)
+print session.query(Employee).\
+            filter(ealias.left>=Employee.left).filter(ealias.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>=Employee.left).filter(ealias.left<=Employee.right).\
+    group_by(ealias.emp).\
+    order_by(ealias.left):
+    print "    " * indentation + str(employee)
+