What is an ORM, how does it work, and how should I use one? [closed]


Object-Relational Mapping (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();

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: 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.
  • It forces you to write MVC 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:

If you want to try an ORM library 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).

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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