title: Can't remove an instance from data base using SQLAlchemy
body: <p>I can't remove an instance from data base using SQLAlchemy.</p>
  
  <p>Here is the code :</p>
  
  <pre><code>from sqlalchemy.orm import sessionmaker, scoped_session
  from zope.sqlalchemy import ZopeTransactionExtension
  from sqlalchemy import create_engine
  from sqlalchemy import MetaData
  from sqlalchemy.ext.declarative import declarative_base
  from pyspatialite import dbapi2 as sqlite
  import transaction
  
  maker = sessionmaker(autoflush=True, autocommit=False, extension=ZopeTransactionExtension())
  DBSession = scoped_session(maker)
  metadata = MetaData()
  Base = declarative_base(metadata=metadata)
  
  filename="/net/my_db.db"
  dburi = 'sqlite:///{0}'.format(filename)
  engine = create_engine(dburi, module=sqlite, echo=False, encoding="utf8")
  metadata.bind = engine
  DBSession.configure(bind=engine)
  
  my_table = metadata.tables['table_name']
  name_col = my_table.columns['col_name']
  my_instance = DBSession.query(my_table).filter(name_col == "toto").first()
  
  DBSession.delete(my_instance)
  </code></pre>
  
  <p>Then I have the following error :</p>
  
  <blockquote>
    <p>UnmappedInstanceError: Class 'sqlalchemy.util._collections.KeyedTuple' is not mapped</p>
  </blockquote>