Editing Object–relational mapping (section)

{{short description|Programming technique}}
{{Distinguish|Object–role modeling}}
{{Use dmy dates|date=June 2019}}
{{More citations needed|date=May 2009}}
'''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 (usually 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 [[Garbage collection (computer science)|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.<ref name="hibernate-orm-overview">
{{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}}
</ref>

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 [[persistence (computer science)|persistent]].<ref name=hibernate-orm-overview />