Object-Relational Mapping

ORM Definition

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

Object-relational mapping (ORM, O/RM, and O/R mapping) in computer science 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.

Overview

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 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.[1]

The heart of the problem is 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.[1]<ref>abc</ref>

Simple Explanation

<ref></ref>A simple answer is that you wrap your tables or stored procedures in classes in your programming language, so that instead of writing SQL statements to interact with your database, you use methods and properties of objects.

In other words, instead of something like this:

String sql = "SELECT ... FROM persons WHERE id = 10"
DbCommand cmd = new DbCommand(connection, sql);
Result res = cmd.Execute();
String name = res[0]["FIRST_NAME"];

you do something like this:

Person p = repository.GetPerson(10);
String name = p.FirstName;

or similar code (lots of variations here.) Some frameworks also put a lot of the code in as static methods on the classes themselves, which means you could do something like this instead:

Person p = Person.Get(10);

Some also implement complex query systems, so you could do this:

Person p = Person.Get(Person.Properties.Id == 10);

The framework is what makes this code possible.

Now, benefits. First of all, you hide the SQL away from your logic code. This has the benefit of allowing you to more easily support more database engines. For instance, MS SQL Server and Oracle has different names on typical functions, and different ways to do calculations with dates, so a query to "get me all persons edited the last 24 hours" might entail different SQL syntax just for those two database engines. This difference can be put away from your logic code.

Additionally, you can focus on writing the logic, instead of getting all the SQL right. The code will typically be more readable as well, since it doesn't contain all the "plumbing" necessary to talk to the database.

ORM Advantage and Disadvantage

Advantage

ORM is a rapidly growing and popular methodology that provides clear advantages to the developer: Eliminates the fragility of coding CRUD statements to persist data to and from the database Allows logic, business rules and validation to be introduced to the data Provides domain objects that are easy to customise and manage in code Hides the differences between various databases or data sources Saves vast amounts of coding effort

References

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