Monday, June 24, 2013

What is an ORM?

Object-relational mapping (ORM, O/RM, and O/R mapping) in computer software is a programming technique for converting data between incompatible type systems in object-oriented programming languages. 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, although some programmers opt to create their own ORM tools.
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.
NHibernate (ORM) solution for the Microsoft .NET platform: it provides a framework for mapping an object-oriented domain model to a traditional relational database. Its purpose is to relieve the developer from a significant portion of relational data persistence-related programming tasks. NHibernate is free as open source software that is distributed under the GNU Lesser General Public License. NHibernate is a port of Hibernate. Below diagram shows the NHibernate integration with database and application.
Comparison with traditional data access techniques
Compared to traditional techniques of exchange between an object-oriented language and a relational database, ORM often reduces the amount of code that needs to be written.
Disadvantages of O/R mapping tools generally stem from the high level of abstraction obscuring what is actually happening under the hood. Also, heavy reliance on ORM software has been cited as a major factor in producing poorly designed databases.

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