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

Someone suggested I use an ORM for a project that I'm designing, but I'm having trouble finding information on what it is or how it works.

Can anyone give me a brief explanation of what an ORM is and how it works and how I should get started using one?

Introduction

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();
     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: 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:

• Java: Hibernate.
• PHP: Propel or Doctrine (I prefer the last one).
• Python: the Django ORM or SQLAlchemy (My favorite ORM library ever).
• C#: NHibernate or Entity Framework

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.

>Can anyone give me a brief explanation...

Sure.

ORM stands for "Object to Relational Mapping" where

• The Object part is the one you use with your programming language ( python in this case )

• The Relational part is a Relational Database Manager System ( A database that is ) there are other types of databases but the most popular is relational ( you know tables, columns, pk fk etc eg Oracle MySQL, MS-SQL )

• And finally the Mapping part is where you do a bridge between your objects and your tables.

In applications where you don't use a ORM framework you do this by hand. Using an ORM framework would allow you do reduce the boilerplate needed to create the solution.

So let's say you have this object.

 class Employee:
      def __init__( self, name ): 
          self.__name = name

       def getName( self ):
           return self.__name

       #etc.

and the table

   create table employee(
          name varcar(10),
          -- etc  
    )

Using an ORM framework would allow you to map that object with a db record automagically and write something like:

   emp = Employee("Ryan")

   orm.save( emp )

And have the employee inserted into the DB.

_{Oops it was not that brief but I hope it is simple enough to catch other articles you read.}

An ORM (Object Relational Mapper) is a piece/layer of software that helps map your code Objects to your database.

Some handle more aspects than others...but the purpose is to take some of the weight of the Data Layer off of the developer's shoulders.

Here's a brief clip from Martin Fowler (Data Mapper):

Patterns of Enterprise Application Architecture Data Mappers

Like all acronyms it's ambiguous, but I assume they mean object-relational mapper -- a way to cover your eyes and make believe there's no SQL underneath, but rather it's all objects;-). Not really true, of course, and not without problems -- the always colorful Jeff Atwood has described ORM as the Vietnam of CS;-). But, if you know little or no SQL, and have a pretty simple / small-scale problem, they can save you time!-)

Object Model is concerned with the following three concepts Data Abstraction Encapsulation Inheritance The relational model used the basic concept of a relation or table. Object-relational mapping (OR mapping) products integrate object programming language capabilities with relational databases.