at_alias = aliased(association_table, name="myJoinedTable")

from sqlalchemy import create_engine, Column, Integer, String, DateTime, ForeignKey, Table, MetaData
from sqlalchemy.orm import sessionmaker, relationship, aliased, backref, registry

mapper_reg = registry()
engine = create_engine('sqlite:///:memory:', echo=False)
metadata = MetaData(bind=engine)
association_table = Table('association', metadata,
                          Column('parent_id', Integer, ForeignKey('things.id')),
                          Column('child_id', Integer, ForeignKey('things.id'))
                          )

things = Table('things', metadata,
               Column('id', Integer, primary_key=True))

class Thing(object):
    pass

mapper_reg.map_imperatively(Thing, things,
       properties={
           'children': relationship(Thing,
                                    secondary=association_table,
                                    primaryjoin=things.c.id == association_table.c.parent_id,
                                    secondaryjoin=things.c.id == association_table.c.child_id)
       })

metadata.create_all(engine)

# Below is where the error occurs
at_alias = aliased(association_table, name="myJoinedTable")