Introduction
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[Object-Relational Mapping][1] (ORM) is a technique that lets you query and manipulate data from a database using an object-oriented paradigm. When talking about ORM, most people are referring to a *library* that implements the Object-Relational Mapping technique, hence the phrase "an ORM".
An ORM library is a completely ordinary library written in your language of choice that encapsulates the code needed to manipulate the data, so you don't use SQL anymore; you interact directly with an object in the same language you're using.
For example, here is 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 library, it would look like this:
book_list = BookTable.query(author="Linus");
The mechanical part is taken care of automatically via the ORM library.
Pros and Cons
-------------
**Using ORM saves a lot of time because:**
- [DRY][2]: You write your data model in only one place, and it's easier to update, maintain, and reuse the code.
- A lot of stuff is done automatically, from database handling to [I18N][12].
- It forces you to write [MVC][3] code, which, in the end, makes your code a little 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, when in reality it's a very powerful and complex one).
- Sanitizing; using prepared statements or transactions are as easy as calling a method.
**Using an ORM library 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 bound to the rest of the application, so you can change it or use it anywhere else.
- It lets you use OOP goodness like data inheritance without a headache.
**But ORM can be a pain:**
- You have to learn it, and ORM libraries are not lightweight tools;
- You have to set it up. Same problem.
- Performance is OK for usual queries, but a SQL master will always do better with his own SQL for big projects.
- It abstracts the DB. While it's OK if you know what's happening behind the scene, it's a trap for new programmers that can write very greedy statements, like a heavy hit in a `for` loop.
How to learn about ORM?
-------------------------
Well, use one. Whichever ORM library you choose, they all use the same principles. There are a lot of ORM libraries around here:
- Java: [Hibernate][4].
- PHP: [Propel][5] or [Doctrine][6] (I prefer the last one).
- Python: the Django ORM or [SQLAlchemy][7] (My favorite ORM library ever).
- C#: [NHibernate][8] or [Entity Framework][9]
If you want to try an ORM library in Web programming, you'd be better off using an entire framework stack like:
- [Symfony][10] (PHP, using Propel or Doctrine).
- [Django][11] (Python, using a internal ORM).
Do not try to write your own ORM, unless you are trying to learn something. This is a gigantic piece of work, and the old ones took a lot of time and work before they became reliable.
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