MySQL and mSQL

Object/Relational Modeling

The most basic question facing the object-oriented developer using a relational database is how to map relational data into objects. Your immediate thought might be to simply map object attributes to fields in a table. Unfortunately, this approach does not create the perfect mapping for several reasons.

• Objects do not store only simple data in their attributes. They may store collections or relationships with other objects.

• Most relational databases—MySQL and mSQL among them—have no way of modeling inheritance.

Rules of Thumb for Object/Relational Modeling

• Each persistent class has a corresponding database table.

• Object fields with primitive datatypes (integers, characters, strings, etc.) map to columns in the associated database table.

• Each row from a database table corresponds to an instance of its associated persistent class.

• Each many-to-many object relationship requires a join table just as database entities with many-to-many relationships require join tables.

• Inheritance is modeled through a one-to-one relationship between the two tables corresponding to the class and subclass.

Think about that address book we talked about earlier. We probably have something like the address and person tables shown in Figure 8.2.

Note

The least apparent issue facing programmers is one of mindset. The basic task of object-oriented access to relational data is to grab that data and immediately instantiate objects. An application should only manipulate data through the objects. ...