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

Question

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

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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 -

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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

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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

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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

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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

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

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