Object-relational Mapping - Overview

Overview

Data management tasks in object-oriented (OO) programming are typically implemented by manipulating objects that are almost always non-scalar values. For example, consider an address book entry that represents a single person along with zero or more phone numbers and zero or more addresses. This could be modeled in an object-oriented implementation by a "Person object" with attributes/fields to hold each data item that the entry comprises: the person's name, a list of phone numbers, and a list of addresses. The list of phone numbers would itself contain "PhoneNumber objects" and so on. The address book entry is treated as a single object by the programming language (it can be referenced by a single variable containing a pointer to the object, for instance). Various methods can be associated with the object, such as a method to return the preferred phone number, the home address, and so on.

However, many popular database products such as structured query language database management systems (SQL DBMS) can only store and manipulate scalar values such as integers and strings organized within tables. The programmer must either convert the object values into groups of simpler values for storage in the database (and convert them back upon retrieval), or only use simple scalar values within the program. Object-relational mapping is used to implement the first approach.

The heart of the problem is translating the logical representation of the objects into an atomized form that is capable of being stored on the database, while somehow preserving the properties of the objects and their relationships so that they can be reloaded as an object when needed. If this storage and retrieval functionality is implemented, the objects are then said to be persistent.