Queue - May/June 2008

Object-Relational Mapping

inheritance, or interfaces are generally more difficult to store and retrieve with reasonable performance.

MAPPING

Inheritance. In the domain object model, inheritance is a relationship between two classes in which one class is a specialization of another. Figure 1 shows a human relations object model in which Full TimeEmployee and Part TimeEmployee are specializations of Employee, which in turn is a specialization of Person.

There are several ways to map this domain object model to relational schema. A single-table inheritance strategy maps all classes in the inheritance hierarchy to a single table that contains a column for each of the fields in any of the classes. In this mapping, slots of the table

Single Ta ble Inheritance Strategy <<table>> PERSON <<PK>>+PERSON_ID : int4 +FIRST_NAME : char varying +LAST_NAME : char varying +MIDDLE_NAME : char varying -HIRE_DATE : date -WEEKLY_SALARY:currency +HOURLY_WAGE:currency -DISCRIMINATOR:char

FIG 2

contain null or default values when the row represents an instance of a class that did not contain a field for that column.

For the single-table strategy to work, the database must contain information that allows the mapping to determine to which class any particular row is mapped. The mapping typically contains the name of a discriminator column and the values of that column that map to each of the possible classes.

For example, the PERSON table (shown in figure 2) might contain a DISCRIMINATOR column whose values are S, E, F, and P, depending on whether the row maps to an instance of Person, Employee, Full TimeEmployee, or PartTimeEmployee, respectively. When retrieving rows from the PERSON table, the provider would always retrieve the value of the DISCRIMINATOR column in order to instantiate the correct class.

A table-per-class inheritance strategy (figure 3) maps each class to its own table and maps each field to a column in the mapped table. All of the data for a single instance would have the same primary-key column value in each of the tables, and the schema would declare foreign-key relationships among the tables. When querying these tables, the provider constructs join queries to determine to which class a row maps. For example, to find all employees hired after a specific date, the query would join the PERSON, EMPLOYEE, PART_TIME_EMPLOYEE, and FULL_TIME_EMPLOYEE tables to determine whether the rows were mapped to Person, Part TimeEmployee, or FullTimeEmployee.

Tab le Per Class Inheritance Strategy <<table>> PERSON <<PK>>+PERSON_ID : int4 +FIRST_NAME : char varying +LAST_NAME : char varying +MIDDLE_NAME : char varying