Object-relational mapping - AbsoluteAstronomy.com

Object-relational mapping (ORM, O/RM, and O/R mapping) in computer software is a programming

Computer programming

Computer programming is the process of designing, writing, testing, debugging, and maintaining the source code of computer programs. This source code is written in one or more programming languages. The purpose of programming is to create a program that performs specific operations or exhibits a...

technique for converting data between incompatible type system

Type system

A type system associates a type with each computed value. By examining the flow of these values, a type system attempts to ensure or prove that no type errors can occur...

s in object-oriented programming languages. This creates, in effect, a "virtual object database

Object database

An object database is a database management system in which information is represented in the form of objects as used in object-oriented programming...

" 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.

Overview

Data management

Data management comprises all the disciplines related to managing data as a valuable resource.- Overview :The official definition provided by DAMA International, the professional organization for those in the data management profession, is: "Data Resource Management is the development and execution...

tasks in object-oriented (OO) programming are typically implemented by manipulating objects

Object (computer science)

In computer science, an object is any entity that can be manipulated by the commands of a programming language, such as a value, variable, function, or data structure...

that are almost always non-scalar

Scalar (computing)

In computing, a scalar variable or field is one that can hold only one value at a time; as opposed to composite variables like array, list, hash, record, etc. In some contexts, a scalar value may be understood to be numeric. A scalar data type is the type of a scalar variable...

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

Attribute (computing)

In computing, an attribute is a specification that defines a property of an object, element, or file. It may also refer to or set the specific value for a given instance of such....

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

SQL

SQL is a programming language designed for managing data in relational database management systems ....

DBMS) can only store and manipulate scalar

Scalar (computing)

values such as integers and strings organized within tables

Table (database)

In relational databases and flat file databases, a table is a set of data elements that is organized using a model of vertical columns and horizontal rows. A table has a specified number of columns, but can have any number of rows...

. 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.

The heart of the problem is translating the logical representation of the objects into an atomized form that is capable of being stored on the database, while somehow preserving the properties of the objects and their relationships so that they can be reloaded as an object when needed. If this storage and retrieval functionality is implemented, the objects are then said to be persistent

Persistence (computer science)

Persistence in computer science refers to the characteristic of state that outlives the process that created it. Without this capability, state would only exist in RAM, and would be lost when this RAM loses power, such as a computer shutdown....

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 O/R mapping tools are in areas where proprietary, or database-specific techniques have been highly optimized.
Most O/R mapping tools do not perform well during bulk deletions of data or joins. Stored procedure

Stored procedure

A stored procedure is a subroutine available to applications that access a relational database system. A stored procedure is actually stored in the database data dictionary.Typical uses for stored procedures include data validation or access control mechanisms...

s may have better performance, but are not portable.

In addition, heavy reliance on ORM software has been pointed to as a major factor in producing poorly designed databases.

Non-SQL databases

Another solution is to use an object-oriented database management system

Object database

An object database is a database management system in which information is represented in the form of objects as used in object-oriented programming...

(OODBMS) or document-oriented database

Document-oriented database

A document-oriented database is a computer program designed for storing, retrieving, and managing document-oriented, or semi structured data, information...

s such as native XML database

XML database

An XML database is a data persistence software system that allows data to be stored in XML format. This data can then be queried, exported and serialized into the desired format.Two major classes of XML database exist:...

s. OODBMS systems 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

Junction table

In database management systems following the relational model, a junction table is a table that contains common fields from two or more tables. It is on the many side of a one-to-many relationship with each of the other tables...

/operations.

Document oriented databases also prevent the user from having to "shred" objects into table rows. Many of these systems also support the XQuery

XQuery

- Features :XQuery provides the means to extract and manipulate data from XML documents or any data source that can be viewed as XML, such as relational databases or office documents....

query language for fast retrieval of large datasets.

Object-oriented databases tend to be used in complex, niche applications. One of the arguments against using an OODBMS is that switching from an SQL DBMS to a purely object-oriented DBMS means that you may lose the capability to create application independent queries for retrieving ad-hoc combinations of data without restriction to access path. 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.

Controversy

Object Relational Mapping technologies have emerged in many application development shops as somewhat divisive.

There are a variety of difficulties that arise when considering how to match an object system to a relational database. These difficulties are refered to as the object-relational impedance mismatch

Object-Relational impedance mismatch

The object-relational impedance mismatch is a set of conceptual and technical difficulties that are often encountered when a relational database management system is being used by a program written in an object-oriented programming language or style; particularly when objects or class definitions...

.

An alternative to implementing ORM is use of the native procedural languages provided with every major database on the market. These can be called from the client using SQL statements

External links

About ORM by Anders Hejlsberg
Anders Hejlsberg
Anders Hejlsberg is a prominent Danish software engineer who co-designed several popular and commercially successful programming languages and development tools...
Mapping Objects to Relational Databases: O/R Mapping In Detail by Scott W. Ambler
Core J2EE Design Pattern: Data Access Objects
Choosing an Object-Relational mapping tool
Five Reasons for using an ORM Tool

The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.

Overview

Comparison with traditional data access techniques

Non-SQL databases

Controversy

See also

External links