Object-Relational Mapping
Object-relational Mapping (ORM, O/RM, and O/R mapping) in computer science is a programming technique for converting data between incompatible type systems in object-oriented programming languages. This creates, in effect, a "virtual object database" that can be used from within the programming language. There are both free and commercial packages available that perform object-relational mapping, although some programmers opt to create their own ORM tools.Object code is written in object-oriented programming (OOP) languages such as Java or C#. ORM converts data between type systems that are unable to coexist within relational databases and OOP languages.<ref name=techopedia>Techopedia,Object-Relational Mapping</ref>
Overview
In object-oriented programming, data management tasks act on object-oriented (OO) objects that are almost always non-scalar values. For example, consider an address book entry that represents a single person along with zero or more phone numbers and zero or more addresses. This could be modeled in an object-oriented implementation by a "Person object" with attributes/fields to hold each data item that the entry comprises: the person's name, a list of phone numbers, and a list of addresses. The list of phone numbers would itself contain "PhoneNumber objects" and so on. The address book entry is treated as a single object by the programming language (it can be referenced by a single variable containing a pointer to the object, for instance). Various methods can be associated with the object, such as a method to return the preferred phone number, the home address, and so on.
However, many popular database products such as structured query language database management systems (SQL DBMS) can only store and manipulate scalar values such as integers and strings organized within tables. The programmer must either convert the object values into groups of simpler values for storage in the database (and convert them back upon retrieval), or only use simple scalar values within the program. Object-relational mapping is used to implement the first approach.[1]
The heart of the problem is translating the logical representation of the objects into an atomized form that is capable of being stored in the database, while preserving the properties of the objects and their relationships so that they can be reloaded as objects when needed. If this storage and retrieval functionality is implemented, the objects are said to be persistent.
ORM Architecture and Framework
Object-relational mapping frameworks were designed to reduce the amount of work needed to develop an object-oriented system that stores data in a relational database. The structure of in-memory objects does not match up well with the structure of data in relational databases, and it can be a slow and repetitive process to develop all the code that is required to move the information between the object space and the relational space. <ref name=Frameworks>introduction to ColdFusion frameworks</ref>
The goal of a persistence framework is to automatically map objects to and from a database seamlessly so that information can be quickly and easily retrieved in object format, and then subsequently stored in its relational structure. This aids in system development and maintenance greatly. The mundane details of writing your own create, read, update, and delete functions are taken care of for you, so you can concentrate on designing and building your application.
Comparison with Traditional Methods
Compared to traditional techniques of exchange between an object-oriented language and a relational database, ORM often reduces the amount of code that needs to be written.<ref>Douglas Barry, Torsten Stanienda, "Solving the Java Object Storage Problem," Computer, vol. 31, no. 11, pp. 33-40, Nov. 1998, http://www2.computer.org/portal/web/csdl/doi/10.1109/2.730734, Excerpt at http://www.service-architecture.com/object-relational-mapping/articles/transparent_persistence_vs_jdbc_call-level_interface.html. Lines of code using O/R are only a fraction of those needed for a call-level interface (1:4). For this exercise, 496 lines of code were needed using the ODMG Java Binding compared to 1,923 lines of code using JDBC.</ref>
Disadvantages of O/R mapping tools generally stem from the high level of abstraction obscuring what is actually happening in the implementation code. Also, heavy reliance on ORM software has been cited as a major factor in producing poorly designed databases.<ref>Josh Berkus, "Wrecking Your Database", Computer, Aug. 2009, http://it.toolbox.com/blogs/database-soup/wrecking-your-database-33298, Webcast at http://www.youtube.com/watch?v=uFLRc6y_O3s</ref>
Simple Example with ORM
String sql = "SELECT ... FROM persons WHERE id = 10"
DbCommand cmd = new DbCommand(connection, sql);
Result res = cmd.Execute();
String name = res[0]["FIRST_NAME"];
you do something like this:
Person p = repository.GetPerson(10);
String name = p.FirstName;
or similar code (lots of variations here.) Some frameworks also put a lot of the code in as static methods on the classes themselves, which means you could do something like this instead:
Person p = Person.Get(10);
Some also implement complex query systems, so you could do this:
Person p = Person.Get(Person.Properties.Id == 10);
ORM Language and Main Tools
This table just includes several well-known object-relational mapping tools regarding to some popular programming languages. It is not up-to-date or all-inclusive. <ref name=listOfORM>Wikipedia,List of Object-relational mapping Software</ref>
| Name | C++ | .NET | Objective-C | Java | Python | Perl | PHP | Ruby |
|---|---|---|---|---|---|---|---|---|
| Several Object-Relational Mapping Tools | LiteSQL | ADO.NET Entity Framework | core Data | jOOQ | Django | DBIx::Class | CakePHP | ActiveRecord |
| ODB | Nhibernate | Enterprise Objects | Apache_Gora | Peewee ORM | Doctrine | Sequel
| ||
| Wt::Dbo | OpenAccess ORM | Hibernate ORM | SQLAlchemy | FuelPHP | DataMapper |
Comparison between ORM tools
The following table lists several well-known object-relational mapping tools <ref name=comparisonOfORM> Wikipedia, Comparison of Object-relational mapping Software</ref>
| ORM Tools | Platform | Availablity | Version |
|---|---|---|---|
| LiteSQL | C++ | Open Source | 0.3.15 / August 10, 2012 |
| ODB | C++ | Dual-licensed(GNU GPL/Proprietary License) | 2.3.0 / October 30, 2013 |
| Wt::Dbo | C++ | Dual-licensed(GNU GPL/Proprietary License) | 3.3.3 / May 27, 2014 |
| ADO.NET Entity Framework | .NET3.5(SP1)+ | Part of .NET 4.0 | EF5.0 |
| Nhibernate | .NET2.0(SP1)+ | Open Source | NH3.3.1 |
| OpenAccess ORM | .NET2.0+ | Free & Commercial | Version 2012.2.816.1 |
| core Data | Objective-C | Commercial | 3.2.0 |
| Enterprise Objects Framework | Objective-C | Commercial | WebObjects 5, released in 2001 |
| jOOQ | Java Virtual Machine | Dual-licensed(ASL 2.0/Commercial) | 3.3.0 / February 14, 2014 |
| Apache_Gora | Java Virtual Machine | Open Source | 0.4 / April 14, 2014 |
| Hibernate ORM | Java Virtual Machine | Open Source | 4.2.5 / August 28, 2013 |
| Django | Python | Open Source | 1.7 / September 2, 2014 |
| Peewee ORM | Python | Open Source | 2.2.3 / April 20, 2014 |
| SQLAlchemy | Python | Open Source | 0.9.4 / March 28, 2014 |
| DBIx::Class | Perl | Open Source | 0.08270 / January 31, 2014 |
| CakePHP | PHP | Open Source | 2.5.4 /September 2, 2014 |
| Doctrine | PHP | Open Source | 2.4/April 8, 2014 |
| FuelPHP | PHP | Open Source | 1.7.2/ July 13, 2014 |
| ActiveRecord | Ruby | Open Source | 4.2.0.beta1/ Aug 20, 2014 |
| Sequel | Ruby | Open Source | 4.14.0 / September 1, 2014 |
| DataMapper | Ruby | Open Source | 1.2.0/October 13, 2011 |
ORM Advantage and Disadvantage
Advantage
ORM is a rapidly growing and popular methodology that provides clear advantages to the developer: Eliminates the fragility of coding CRUD statements to persist data to and from the database Allows logic, business rules and validation to be introduced to the data Provides domain objects that are easy to customise and manage in code Hides the differences between various databases or data sources Saves vast amounts of coding effort
- Simplified development because it automates object-to-table and table-to-object conversion, resulting in lower development and maintenance costs
- Less code compared to embedded SQL and handwritten stored procedures
- Transparent object caching in the application tier, improving system performance
- An optimized solution making an application faster and easier to maintain<ref name=techopedia/>
Disadvantage
- ORM does not perform well and that stored procedures might be a better solution.
- ORM dependence may result in poorly-designed databases in certain circumstances.<ref name=techopedia/>
- ORM is slower than hand-crafted persistence code that directly interacts with a database API such as JDBC or ADO.NET.
- Most ORM systems have unsatisfactory support for revisions to the architecture of a system after a system has been mapped.
- ORM has yet to see widespread adoption in everyday industrial operations.<ref name=abc>Barnes, Jeffrey M. Object-Relational Mapping as a Persistence Mechanism for Object-Oriented Applications. Diss. Macalester College, 2007.WebCast at http://digitalcommons.macalester.edu/mathcs_honors/6/</ref>
Database Support
See Also
References
<references></references>
External Links
- Ruby on Rails official website – rubyonrails.org
- Railscasts.com – these screencasts are short and focus on Ruby on Rails technique
- Rails source code