domain of concrete inheritance, and we must build a special mapper against
``Employee`` in order to achieve this.
-.. topic:: Mappers can always SELECT
-
- In SQLAlchemy, a mapper for a class always has to refer to some
- "selectable", which is normally a :class:`_schema.Table` but may also refer to any
- :func:`_expression.select` object as well. While it may appear that a "single table
- inheritance" mapper does not map to a table, these mappers in fact
- implicitly refer to the table that is mapped by a superclass.
-
To modify our concrete inheritance example to illustrate an "abstract" base
that is capable of polymorphic loading,
we will have only an ``engineer`` and a ``manager`` table and no ``employee``