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Object–relational mapping (ORM, O/RM, and O/R mapping tool) in
computer science Computer science is the study of computation, information, and automation. Computer science spans Theoretical computer science, theoretical disciplines (such as algorithms, theory of computation, and information theory) to Applied science, ...
is a programming technique for converting data between a
relational database A relational database (RDB) is a database based on the relational model of data, as proposed by E. F. Codd in 1970. A Relational Database Management System (RDBMS) is a type of database management system that stores data in a structured for ...
and the memory (usually the heap) of an object-oriented programming language. This creates, in effect, a virtual object database that can be used from within the programming language. In
object-oriented programming Object-oriented programming (OOP) is a programming paradigm based on the concept of '' objects''. Objects can contain data (called fields, attributes or properties) and have actions they can perform (called procedures or methods and impl ...
, data-management tasks act on objects that combine scalar values into objects. 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 an attribute/field 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. Each such 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 methods to return the preferred phone number, the home address, and so on. By contrast, relational databases, such as SQL, group scalars into
tuples In mathematics, a tuple is a finite sequence or ''ordered list'' of numbers or, more generally, mathematical objects, which are called the ''elements'' of the tuple. An -tuple is a tuple of elements, where is a non-negative integer. There is on ...
, which are then enumerated in tables. Tuples and objects have some general similarity, in that they are both ways to collect values into named fields such that the whole collection can be manipulated as a single compound entity. They have many differences, though, in particular: lifecycle management (row insertion and deletion, versus garbage collection or reference counting), references to other entities (object references, versus foreign key references), and inheritance (non-existent in relational databases). As well, objects are managed on-heap and are under full control of a single process, while database tuples are shared and must incorporate locking, merging, and retry. Object–relational mapping provides automated support for mapping tuples to objects and back, while accounting for all of these differences. The heart of the problem involves 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.


Overview

Implementation-specific details of storage drivers are generally wrapped in an API in the programming language in use, exposing methods to interact with the storage medium in a way which is simpler and more in line with the paradigms of surrounding code. The following is a simple example, written in C# code, to execute a query written in SQL using a database engine. var sql = "SELECT id, first_name, last_name, phone, birth_date, sex, age FROM persons WHERE id = 10"; var result = context.Persons.FromSqlRaw(sql).ToList(); var name = result "first_name"]; In contrast, the following makes use of an ORM-job API which makes it possible to write code that naturally makes use of the features of the language. var person = repository.GetPerson(10); var firstName = person.GetFirstName(); The case above makes use of an object representing the storage repository and methods of that object. Other frameworks might provide code as static methods, as in the example below, and yet other methods may not implement an object-oriented system at all. Often the choice of paradigm is made for the best fit of the ORM into the surrounding language's design principles. var person = Person.Get(10);


Comparison with traditional data access techniques

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. Disadvantages of ORM tools generally stem from the high level of Database abstraction layer, abstraction obscuring what is actually happening in the implementation code.


Object-oriented databases

Another approach is to use an object-oriented database management system (OODBMS) or
document-oriented database A document-oriented database, or document store, is a computer program and data storage system designed for storing, retrieving and managing document-oriented information, also known as semi-structured data. Document-oriented databases are one ...
s such as native
XML database An XML database is a data persistence software system that allows data to be specified, and stored, in XML format. This data can be queried, transformed, exported and returned to a calling system. XML databases are a flavor of document-oriented ...
s that provide more flexibility in data modeling. OODBMSs are databases designed specifically for working with object-oriented values. Using an OODBMS eliminates the need for converting data to and from its SQL form, as the data is stored in its original object representation and relationships are directly represented, rather than requiring join tables/operations. The equivalent of ORMs for
document-oriented database A document-oriented database, or document store, is a computer program and data storage system designed for storing, retrieving and managing document-oriented information, also known as semi-structured data. Document-oriented databases are one ...
s are called object-document mappers (ODMs). Document-oriented databases also prevent the user from having to "shred" objects into table rows. Many of these systems also support the
XQuery XQuery (XML Query) is a query language and functional programming language designed to query and transform collections of structured and unstructured data, primarily in the form of XML. It also supports text data and, through implementation-sp ...
query language to retrieve datasets. Object-oriented databases tend to be used in complex, niche applications. One of the arguments against using an OODBMS is that it may not be able to execute ad-hoc, application-independent queries. For this reason, many programmers find themselves more at home with an object-SQL mapping system, even though most object-oriented databases are able to process SQL queries to a limited extent. Other OODBMS provide replication to SQL databases, as a means of addressing the need for ad-hoc queries, while preserving well-known query patterns.


Challenges

A variety of difficulties arise when considering how to match an object system to a relational database. These difficulties are referred to as the object–relational impedance mismatch. An alternative to implementing ORM is use of the native procedural languages provided with every major database. These can be called from the client using SQL statements. The Data Access Object (DAO) design pattern is used to abstract these statements and offer a lightweight object-oriented interface to the rest of the application. ORMs are limited to their predefined functionality, which may not cover all edge cases or database features. They usually mitigate this limitation by providing users with an interface to write raw queries, such as Django ORM.


See also

* List of object–relational mapping software * Comparison of object–relational mapping software * AutoFetch – automatic query tuning * Common Object Request Broker Architecture (CORBA) * Object database * Object persistence *
Object–relational database An object–relational database (ORD), or object–relational database management system (ORDBMS), is a database management system (DBMS) similar to a relational database, but with an object-oriented database model: objects, classes and inherit ...
* Object–relational impedance mismatch *
Relational model The relational model (RM) is an approach to managing data using a structure and language consistent with first-order predicate logic, first described in 1969 by English computer scientist Edgar F. Codd, where all data are represented in terms of t ...
** SQL (Structured Query Language) * Java Data Objects (JDO) * Java Persistence API (JPA), now Jakarta Persistence * Service Data Objects * Entity Framework * Active record pattern * Data mapper pattern * Single Table Inheritance


References


External links


About ORM
by
Anders Hejlsberg Anders Hejlsberg (; ; born 2 December 1960) is a Denmark, Danish software engineer who co-designed several programming languages and development tools. He was the original author of Turbo Pascal and the chief architect of Delphi (programming lang ...

Mapping Objects to Relational Databases: O/R Mapping In Detail
by Scott W. Ambler {{DEFAULTSORT:Object-Relational Mapping Data mapping Articles with example C Sharp code