The Declarative system is the typically used system provided by the SQLAlchemy
ORM in order to define classes mapped to relational database tables. However,
as noted in Classical Mappings, Declarative is in fact a series of
extensions that ride on top of the SQLAlchemy mapper() construct.
While the documentation typically refers to Declarative for most examples,
the following sections will provide detailed information on how the
Declarative API interacts with the basic mapper() and Core Table
systems, as well as how sophisticated patterns can be built using systems
such as mixins.