The document discusses using database constraints to improve reliability, speed, and security of a relational database. It recommends using simple constraints like not null, unique indices, and foreign keys to define the database interface. More complex constraints like polymorphic relationships can be handled by the Polymorpheus gem, which takes care of validation and uniqueness through migrations. The overall message is that properly setting up database constraints through tools like the gem can keep application logic clean while establishing a robust database structure.