What Is ORM ?

Detailed info:

An ORM (Object-Relational Mapping) is a tool that lets you query and manipulate data from a database using an object paradigm.
It's a completely ordinary library written in your language that encapsulates the code needed to manipulate the data, so you don't use SQL anymore, but directly an object of your language.
E.G., a completely imaginary case with a pseudo language :
You have a book class, you want to retrieve all the books of which the author is "Linus". Manually, you would do something like that :

book_list = new List();sql = "SELECT book FROM library WHERE author = 'Linus'";data = query(sql); // I over simplify ...while (row = data.next()){     book = new Book();     book.setAuthor(row.get('author');     book_list.add(book);}

With an ORM, it would look like that :

book_list = BookTable.query(author="Linus");

The mechanical part is taken care automatically by the ORM.
Pros and cons
Using an ORM save a lot of time because :

• DRY : You write your data model in only one place, it's easier to update, maintain and reuse the code.
• A lot of stuff is done automatically, from the database handling to I18N.
• It forces you to write MVC code, and in the end your app is cleaner.
• You don't have to write poorly formed SQL (most Web programmers really suck at it, because SQL is treated like a "sub" language whereas it's a very powerful and complex one)
• Sanitizing, using prepared statements or transactions are as easy as calling a method.

Using an ORM is more flexible because :

• It fits in your natural way of coding (it's your language !)
• It abstracts the DB system, so you can change it whenever you want.
• The model is weakly binded to the rest of the app, so you can change it or use it anywhere else.
• It let you use OOP goodness like data inheritance without head ache.

But ORM can be a pain :

• You have to learn it, and they are not lightweight tools;
• You have to set it up. Same problem.
• Performances are ok for usual queries, but a SQL master will always do better with his little hands for the big dirty works.
• It abstracts the DB. While it's ok if you know what's happening behind the scene, it's a trap for the noobs that can write very greedy statements, like a heavy hit in a for loop...

How to learn about them ?
Well, use one. What ever the one you choose, they all use the same principles. There are a lot of ORMs around here :

• Java : Hibernate.
• PHP : Propel or Doctrine (I prefer the last one).
• Python : the Django ORM or SQLAlchemy (The last one is my favorite ORM ever).

If you want to try an ORM in Web programming, you'd be better off using an entire framework stack like :

• Symfony (PHP, using Propel or Doctrine)
• Django (Python, using a internal ORM)