2.3. Theory Object-Relational Mapping¶
ORM - Object-relational mapping
Converts (map) between objects in code and database tables (relations)
Declarative - First define model, which then maps to the database tables
Object–relational mapping (ORM, O/RM, and O/R mapping tool) in computer science is a programming technique for converting data between incompatible type systems using object-oriented programming languages. This creates, in effect, a 'virtual object database' 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 construct their own ORM tools 1.
In object-oriented programming, data-management tasks act on objects that are almost always non-scalar 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 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 1.
By contrast, many popular database products such as SQL database management systems (DBMS) are not object-oriented and can only store and manipulate scalar values such as integers and strings organized within tables. 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 implements the first approach 2.
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 1.
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. 3
Disadvantages of ORM tools generally stem from the high level of 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. 4
What is Object/Relational Mapping?. Hibernate Overview. JBOSS Hibernate. Retrieved: 2011-04-19. URL: https://hibernate.org/orm/what-is-an-orm/
Barry, Douglas and Stanienda, Torsten. Solving the Java Object Storage Problem. Computer, vol. 31, no. 11, pp. 33-40, Nov. 1998, http://www2.computer.org/portal/web/csdl/doi/10.1109/2.730734 , Excerpt at http://www.service-architecture.com/object-relational-mapping/articles/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.
Lorenz, M and Rudolph, J.P. and Hesse, G. and Uflacker, M. and Plattner, H. Object–Relational Mapping Revisited - A Quantitative Study on the Impact of Database Technology on O/R Mapping Strategies. Hawaii International Conference on System Sciences (HICSS), 4877-4886 (DOI:10.24251/hicss.2017.592). Year: 2017.