Object Relational Mapping

A PersistenceLayer connecting objects in an object-oriented system to data stored in a relational database. Most commonly, an SQL database is used (though strictly speaking, SQL does not implement the RelationalModel -- see SqlFlaws and RelationalLanguage for more details).

Doesn't all the work required in implementing ORM violate OnceAndOnlyOnce? I never understood why OO languages fear instead of incorporate a database

Using an ObjectRelationalMapping (ORM) allows one to cleanly apply object-oriented design, analysis, and programming techniques while hiding the specifics of dealing with the relational system.

There are commercial and OpenSource ORMs that comply to the ODMG and JDO standard APIs for object persistence. Programming against such a mapping is not different from programming against an ObjectOrientedDatabase.

Using such a ORM may be a good compromise between OO designers/architects insisting on pure object orientation without any data knowledge, and DBAs/managers emphasizing the shortcomings of OODBMSs.

-- ThomasMahler

A PrevalenceLayer is another option to forget database hassle.

See also the following:

DatabaseImpedanceMismatch, ObjectRelationalImpedanceMismatch:: Discussions of incompatibilities between the design of databases and application host languages. For example, set operations and null values are not supported in many host languages.
LetsUseAnObjectOrientedDatabase:: Discussion of business's choice of RelationalDatabases instead of Object Databases.
CrossingChasms:: Pattern Language for mapping objects to RelationalDatabase tables.
DataTransferObjectInjection (A (common?) security issue when combined with RemoteObjectServices?)
http://www.ambysoft.com/essays/mappingObjects.html : ScottAmbler's papers on mapping objects to RelationalDatabase tables.
http://www.sce-tech.com/architecture/persistence_layers.html : Article on persistence layer architecture, including design principles and anti-patterns.
RelationalDatabaseAccessLayer:: Mapping RelationalDatabase to procedural code.
http://www.SimpleORM.org/ORMTools.html : Another review of many different tools and issues, organized by the structure of the API (CMP, JDO, Generalized...).
http://www.visual-paradigm.com/dbva.php : A full featured Object Relational Mapping (ORM) toolset helps realizing UML design to database implementation.

Implementations:

Would be nice to separate these by language...

Java Skyline Database page http://www.javaskyline.com/database.html

Lists over 50 Object-SQL Mapping Tools, - The scope of each tool - A list of other Java database resources and - A list of other object-SQL tool lists, including the:

ObjectRelationalMappingExperiences using these products (or your own) could also offer some insight.

ObjectRelationalToolComparison contains an extensive comparison of popular O/R tools.

ObjectRelationalBridge (OJB):: OpenSource project providing ODMG and JDO compliant APIs. http://db.apache.org/ojb/
FireStorm/DAO:: generates DataAccessObjects (DAOs) directly from database schemas.

SQL Orm:: Simple low level SQL-based ORM with easy refactoring possibilities http://sqlorm.sourceforge.net/database_refactoring_made_easy.html

TopLink:: ObjectRelationalMapping product for Java on web servers. (SmalltalkLanguage too?)
EnterpriseObjectsFramework (EOF):: The framework used in WebObjects, from AppleComputer (now pure Java in WebObjectsFive). Documentation can be found at http://developer.apple.com/techpubs/webobjects/webobjects.html

http://www.SimpleORM.org A very Simple, light weight ORM. No generation, byte code post processing or even an XML file to configure. But fast and effective.

http://www.sce-tech.com/research/persistence.html: Persistence technology including 'active' relationships and path expressions. Originally conceived from the EOF vision.
CastorFramework:: The OpenSource framework which uses CastorJDO to convert java objects to SQL or XML data.
AtgRepositoryFramework?:: The framework used in AtgDynamo, from Art Technology Group. Handles Java, XML, LDAP, SQL, sub classes of items (i.e. you can have a RepositoryItem? representing Person and subclass that with more information to make a Manager), instance inheritance (doing sort of the same thing, but dynamically), and property derivation (get the name from the subclass if it exists, otherwise get it from the superclass).
CayenneFramework:: The OpenSource framework and GUI tool that provides functionality similar to EnterpriseObjectsFramework.
JaxorFramework:: Generates persistence layer as well as objects based upon xml metadata.
JdoGenie:: Fast JDO implementation for JDBC databases.
ProductivityEnvironmentForJava:: JDO implementation for JDBC RelationalDatabase systems.
ClassDbi:: OpenSource Perl object to database mapping layer.
HiberNate:: OpenSource Java http://hibernate.sourceforge.net
ActiveRecord:: OpenSource Ruby framework http://activerecord.rubyonrails.org/show/HomePage
SQLObject:: OpenSource Python framework http://sqlobject.org/
EZPDO:: OpenSource ORM Framework for PHP5 http://www.ezpdo.net
Picasso:: OpenSource .NET http://picasso.codeplex.com
Gentle.NET:: OpenSource framework for .NET written in C# based on ScottAmbler's papers. Supports PostgreSQL, MySQL, MS SQL Server and SQLite. http://freshmeat.net/projects/gentle (see http://www.mertner.com/projects/gentle for a tutorial).
Genome:: O/R Mapper for .NET and SQL Server/Oracle (more information and eval download: http://www.genom-e.com); free community license
AtomsFramework:: OpenSource framework for VB6 and .NET at http://jcframework.sourceforge.net

ChrisDate and HughDarwen have much to say on this in TheThirdManifesto, ISBN 0-201-70928-7 -- ChanningWalton

Any opinions on the JavaLayeredFrameworks approach outlined by ToddLauinger? Has anyone tried something like this? -- BernardFarrell

Class structures don't map properly to relations. This is due to what Date calls the FirstGreatBlunder, that is, mapping objects or classes to SQL tables, instead of to user-defined data types.

A sequence, as might be implemented by linked list, must be represented either by precedence (as in CREATE TABLE sequence (father INTEGER, child INTEGER);) or by a specific type... unfortunately SQL doesn't support user-defined types. Also one can have a "SequenceOrder" attribute that is a real number or integer. But linked lists are not an OO concept. Often times a time-stamp or auto-generated ID's can be used if it is to capture a temporal sequence.