Object-relational mapping

In computer science, Object-relational mapping (ORM, O/RM, and O/R mapping tool) is a programming technique for converting data (computing) between incompatible type systems in object-oriented programming languages.

Description

This creates, in effect, a "virtual object database" that can be used from within the programming language.

There are both free and commercial packages available that perform object-relational mapping; also, some programmers construct their own ORM tools.

In object-oriented programming, data-management tasks act on object-oriented (OO) 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 SQL database management systems (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 implements the first approach.

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

See also

• Active record pattern
• AutoFetch – automatic query tuning
• Common Object Request Broker Architecture (CORBA)
• Comparison of object-relational mapping software
• Data (computing)
• Data type
• Entity Framework
• Java Data Objects
• List of object-relational mapping software
• Object database
• Object-oriented programming
• Object persistence
• Object-relational database
• Object-relational impedance mismatch
• Relational model
• Service Data Objects
• Type system

External links

• Object-relational mapping @ Wikipedia