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{{short description|Programming technique}}▼
{{Distinguish|Object–role modeling}}
{{Use dmy dates|date=June 2019}}
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'''Object–relational mapping''' ('''ORM''', '''O/RM''', and '''O/R mapping tool''') in [[computer science]] is a [[Computer programming|programming]] technique for converting data between a [[relational database]] and the [[
▲{{short description|Programming technique}}
▲'''Object–relational mapping''' ('''ORM''', '''O/RM''', and '''O/R mapping tool''') in [[computer science]] is a [[Computer programming|programming]] technique for converting data between a [[relational database]] and the [[Memory_management#HEAP|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]], [[data management|data-management]] tasks act on [[object (computer science)|object]]s that combine [[scalar (computing)|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 (computer science)|object]]" with an [[attribute (computing)|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 [[Method (computer programming)|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]], which are then enumerated in [[Table (database)|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 [[
{{cite web |title=What is Object/Relational Mapping? |url=http://www.hibernate.org/about/orm |access-date=27 January 2022 |work=Hibernate Overview |publisher=JBOSS Hibernate |language=en-US}}
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==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.<ref>Douglas Barry, Torsten Stanienda, "Solving the Java Object Storage Problem," Computer, vol. 31, no. 11, pp. 33-40, Nov. 1998, [https://www.computer.org/csdl/magazine/co/1998/11/ry033/13rRUxC0SRY. Excerpt at https://www.service-architecture.com/articles/object-relational-mapping/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 ORM tools generally stem from the high level of [[Database abstraction layer|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, https://www.toolbox.com/tech/data-management/blogs/wrecking-your-database-080509/</ref>
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