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<title>Object–relational mapping</title>

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<span id="openzim-page-title" class="mw-page-title-main"><span class="mw-page-title-main">Object–relational mapping</span></span>

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</style><div role="note" class="hatnote navigation-not-searchable">Not to be confused with <a href="Object%E2%80%93role_modeling" title="Object–role modeling">Object–role modeling</a>.</div>

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<p><b>Object–relational mapping</b> (<b>ORM</b>, <b>O/RM</b>, and <b>O/R mapping tool</b>) in <a href="Computer_science" title="Computer science">computer science</a> is a <a href="Computer_programming" title="Computer programming">programming</a> technique for converting data between a <a href="Relational_database" title="Relational database">relational database</a> and the memory (usually the <a href="Memory_management#HEAP" title="Memory management">heap</a>) of an <a href="Object-oriented_programming" title="Object-oriented programming">object-oriented</a> programming language. This creates, in effect, a virtual <a href="Object_database" title="Object database">object database</a> that can be used from within the programming language.

</p><p>In <a href="Object-oriented_programming" title="Object-oriented programming">object-oriented programming</a>, <a href="Data_management" title="Data management">data-management</a> tasks act on <a href="Object_(computer_science)" title="Object (computer science)">objects</a> that combine <a href="Scalar_(computing)" class="mw-redirect" title="Scalar (computing)">scalar</a> 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 <a href="Object_(computer_science)" title="Object (computer science)">object</a>" with an <a href="Attribute_(computing)" title="Attribute (computing)">attribute/field</a> 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 <a href="Method_(computer_programming)" title="Method (computer programming)">methods</a> can be associated with the object, such as methods to return the preferred phone number, the home address, and so on.

</p><p>By contrast, relational databases, such as <a href="SQL" title="SQL">SQL</a>, group scalars into <a href="Tuples" class="mw-redirect" title="Tuples">tuples</a>, which are then enumerated in <a href="Table_(database)" title="Table (database)">tables</a>. 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 <a href="Garbage_collection_(computer_science)" title="Garbage collection (computer science)">garbage collection</a> or <a href="Reference_counting" title="Reference counting">reference counting</a>), 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.<sup id="cite_ref-hibernate-orm-overview_1-0" class="reference"><a href="#cite_note-hibernate-orm-overview-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup>

</p><p>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 <a href="Persistence_(computer_science)" title="Persistence (computer science)">persistent</a>.<sup id="cite_ref-hibernate-orm-overview_1-1" class="reference"><a href="#cite_note-hibernate-orm-overview-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup>

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<div class="mw-heading mw-heading2"><h2 id="Overview">Overview</h2></div>

<p>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.

</p><p>The following is a simple example, written in <a href="C_Sharp_(programming_language)" title="C Sharp (programming language)">C#</a> code, to execute a query written in <a href="SQL" title="SQL">SQL</a> using a database engine.

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<div class="mw-highlight mw-highlight-lang-csharp mw-content-ltr" dir="ltr"><pre><span class="kt">var</span><span class="w"> </span><span class="n">sql</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="s">"SELECT id, first_name, last_name, phone, birth_date, sex, age FROM persons WHERE id = 10"</span><span class="p">;</span>

<span class="kt">var</span><span class="w"> </span><span class="n">result</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">context</span><span class="p">.</span><span class="n">Persons</span><span class="p">.</span><span class="n">FromSqlRaw</span><span class="p">(</span><span class="n">sql</span><span class="p">).</span><span class="n">ToList</span><span class="p">();</span>

<span class="kt">var</span><span class="w"> </span><span class="n">name</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">result</span><span class="p">[</span><span class="m">0</span><span class="p">][</span><span class="s">"first_name"</span><span class="p">];</span>

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<p>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.

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<div class="mw-highlight mw-highlight-lang-csharp mw-content-ltr" dir="ltr"><pre><span class="kt">var</span><span class="w"> </span><span class="n">person</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">repository</span><span class="p">.</span><span class="n">GetPerson</span><span class="p">(</span><span class="m">10</span><span class="p">);</span>

<span class="kt">var</span><span class="w"> </span><span class="n">firstName</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">person</span><span class="p">.</span><span class="n">GetFirstName</span><span class="p">();</span>

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<p>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.

</p>

<div class="mw-highlight mw-highlight-lang-csharp mw-content-ltr" dir="ltr"><pre><span class="kt">var</span><span class="w"> </span><span class="n">person</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">Person</span><span class="p">.</span><span class="n">Get</span><span class="p">(</span><span class="mi">10</span><span class="p">);</span>

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<div class="mw-heading mw-heading2"><h2 id="Comparison_with_traditional_data_access_techniques">Comparison with traditional data access techniques</h2></div>

<p>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.<sup id="cite_ref-2" class="reference"><a href="#cite_note-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup>

</p><p>Disadvantages of ORM tools generally stem from the high level of <a href="Database_abstraction_layer" title="Database abstraction layer">abstraction</a> obscuring what is actually happening in the implementation code.

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<div class="mw-heading mw-heading2"><h2 id="Object-oriented_databases">Object-oriented databases</h2></div>

<p>Another approach is to use an <a href="Object_database" title="Object database">object-oriented database management system</a> (OODBMS) or <a href="Document-oriented_database" title="Document-oriented database">document-oriented databases</a> such as native <a href="XML_database" title="XML database">XML databases</a> 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 <a href="Junction_table" class="mw-redirect" title="Junction table">join tables</a>/operations. The equivalent of ORMs for <a href="Document-oriented_database" title="Document-oriented database">document-oriented databases</a> are called object-document mappers (ODMs).

</p><p>Document-oriented databases also prevent the user from having to "shred" objects into table rows. Many of these systems also support the <a href="XQuery" title="XQuery">XQuery</a> query language to retrieve datasets.

</p><p>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.

</p>

<div class="mw-heading mw-heading2"><h2 id="Challenges">Challenges</h2></div>

<p>A variety of difficulties arise when considering how to match an object system to a relational database. These difficulties are referred to as the <a href="Object%E2%80%93relational_impedance_mismatch" title="Object–relational impedance mismatch">object–relational impedance mismatch</a>.<sup id="cite_ref-3" class="reference"><a href="#cite_note-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup>

</p><p>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 <a href="Data_access_object" title="Data access object">Data Access Object</a> (DAO) design pattern is used to abstract these statements and offer a lightweight object-oriented interface to the rest of the application.<sup id="cite_ref-4" class="reference"><a href="#cite_note-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup>

</p><p>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.<sup id="cite_ref-5" class="reference"><a href="#cite_note-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup>

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<ul><li><a href="List_of_object%E2%80%93relational_mapping_software" title="List of object–relational mapping software">List of object–relational mapping software</a></li>

<li><a href="Comparison_of_object%E2%80%93relational_mapping_software" title="Comparison of object–relational mapping software">Comparison of object–relational mapping software</a></li>

<li><a href="AutoFetch" title="AutoFetch">AutoFetch</a> – automatic query tuning</li>

<li><a href="Common_Object_Request_Broker_Architecture" title="Common Object Request Broker Architecture">Common Object Request Broker Architecture</a> (CORBA)</li>

<li><a href="Object_database" title="Object database">Object database</a></li>

<li><a href="Object_persistence" class="mw-redirect" title="Object persistence">Object persistence</a></li>

<li><a href="Object%E2%80%93relational_database" title="Object–relational database">Object–relational database</a></li>

<li><a href="Object%E2%80%93relational_impedance_mismatch" title="Object–relational impedance mismatch">Object–relational impedance mismatch</a></li>

<li><a href="Relational_model" title="Relational model">Relational model</a>

<ul><li><a href="SQL" title="SQL">SQL</a> (Structured Query Language)</li></ul></li>

<li><a href="Java_Data_Objects" title="Java Data Objects">Java Data Objects</a> (JDO)</li>

<li><a href="Java_Persistence_API" class="mw-redirect" title="Java Persistence API">Java Persistence API</a> (JPA), now <a href="Jakarta_Persistence" title="Jakarta Persistence">Jakarta Persistence</a></li>

<li><a href="Service_Data_Objects" title="Service Data Objects">Service Data Objects</a></li>

<li><a href="Entity_Framework" title="Entity Framework">Entity Framework</a></li>

<li><a href="Active_record_pattern" title="Active record pattern">Active record pattern</a></li>

<li><a href="Data_mapper_pattern" title="Data mapper pattern">Data mapper pattern</a></li>

<li><a href="Single_Table_Inheritance" title="Single Table Inheritance">Single Table Inheritance</a></li></ul>

<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>

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</style><cite class="citation web cs1"><a rel="nofollow" class="external text" href="http://www.hibernate.org/about/orm">"What is Object/Relational Mapping?"</a>. <i>Hibernate Overview</i>. JBOSS Hibernate<span class="reference-accessdate">. Retrieved <span class="nowrap">27 January</span> 2022</span>.</cite></span>

</li>

<li id="cite_note-2"><span class="mw-cite-backlink"><b><a href="#cite_ref-2">^</a></b></span> <span class="reference-text"><cite id="CITEREFBarry,_D.Stanienda,_T.1998" class="citation journal cs1 cs1-prop-vanc-accept">Barry, D., Stanienda, T. (1998). <a rel="nofollow" class="external text" href="https://www.service-architecture.com/articles/object-relational-mapping/transparent-persistence-vs-jdbc-call-level-interface.html">"Solving the Java object storage problem"</a>. <i>Computer</i>. <b>31</b> (11). Institute of Electrical and Electronics Engineers (IEEE): <span class="nowrap">33–</span>40. <a href="Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<a rel="nofollow" class="external text" href="https://doi.org/10.1109%2F2.730734">10.1109/2.730734</a>. <q>For this exercise, 496 lines of code were needed using the ODMG Java Binding compared to 1,923 lines of code using JDBC.</q></cite></span>

</li>

<li id="cite_note-3"><span class="mw-cite-backlink"><b><a href="#cite_ref-3">^</a></b></span> <span class="reference-text"><a rel="nofollow" class="external text" href="https://www.semanticscholar.org/paper/Object-Relational-Mapping-Revisited-A-Quantitative-Lorenz-Rudolph/708ac5e798b7e45b949d42e2f872549a3612e1e2">Object–Relational Mapping Revisited - A Quantitative Study on the Impact of Database Technology on O/R Mapping Strategies. M Lorenz, JP Rudolph, G Hesse, M Uflacker, H Plattner. Hawaii International Conference on System Sciences (HICSS), 4877-4886</a> (DOI:10.24251/hicss.2017.592)</span>

</li>

<li id="cite_note-4"><span class="mw-cite-backlink"><b><a href="#cite_ref-4">^</a></b></span> <span class="reference-text"><cite id="CITEREFFeuersteinBill_Pribyl1997" class="citation web cs1">Feuerstein, Steven; Bill Pribyl (September 1997). <a rel="nofollow" class="external text" href="http://docstore.mik.ua/orelly/oracle/prog2/ch18_05.htm">"Oracle PL/SQL Programming"</a>. 18.5 Modifying Persistent Objects<span class="reference-accessdate">. Retrieved <span class="nowrap">23 August</span> 2011</span>.</cite><span class="cs1-maint citation-comment"><code class="cs1-code">{{cite web}}</code>: CS1 maint: location (link)</span></span>

</li>

<li id="cite_note-5"><span class="mw-cite-backlink"><b><a href="#cite_ref-5">^</a></b></span> <span class="reference-text"><cite class="citation web cs1"><a rel="nofollow" class="external text" href="https://docs.djangoproject.com/en/5.1/topics/db/sql/">"Performing raw SQL queries | Django documentation"</a>. <i>Django Project</i><span class="reference-accessdate">. Retrieved <span class="nowrap">8 September</span> 2024</span>.</cite></span>

</li>

</ol></div></div>

<div class="mw-heading mw-heading2"><h2 id="External_links">External links</h2></div>

<ul><li><a rel="nofollow" class="external text" href="http://www.artima.com/intv/abstract3.html">About ORM</a> by <a href="Anders_Hejlsberg" title="Anders Hejlsberg">Anders Hejlsberg</a></li>

<li><a rel="nofollow" class="external text" href="http://www.agiledata.org/essays/mappingObjects.html">Mapping Objects to Relational Databases: O/R Mapping In Detail</a> by <a href="Scott_W._Ambler" class="mw-redirect" title="Scott W. Ambler">Scott W. Ambler</a></li></ul>

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</style><div id="Database414" style="font-size:114%;margin:0 4em"><a href="Database" title="Database">Database</a></div></th></tr><tr><th scope="row" class="navbox-group" style="width:1%">Main</th><td class="navbox-list-with-group navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Database#Major_database_usage_requirements" title="Database">Requirements</a></li>

<li><a href="Database_theory" title="Database theory">Theory</a></li>

<li><a href="Database_object" title="Database object">Database object</a></li>

<li><a href="Database_model" title="Database model">Models</a></li>

<li><a href="Database_management_system" class="mw-redirect" title="Database management system">Database management system</a></li>

<li><a href="Database_machine" title="Database machine">Machine</a></li>

<li><a href="Database_engine" title="Database engine">Engine</a></li>

<li><a href="Database_server" title="Database server">Server</a></li>

<li><a href="Database_application" title="Database application">Application</a></li>

<li><a href="Database_connection" title="Database connection">Connection</a>

<ul><li><a href="Datasource" title="Datasource">datasource</a></li>

<li><a href="Database_administrator" title="Database administrator">Administrator</a></li>

<li><a href="Synonym_(database)" title="Synonym (database)">Synonym</a></li>

<li>Types</li>

<li><a href="Comparison_of_database_tools" class="mw-redirect" title="Comparison of database tools">Tools</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="Database#Database_languages" title="Database">Languages</a></th><td class="navbox-list-with-group navbox-list navbox-even hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Data_definition_language" title="Data definition language">Data definition</a></li>

<li><a href="Data_manipulation_language" title="Data manipulation language">Data manipulation</a></li>

<li><a href="Query_language" title="Query language">Query</a>

<ul><li><a href="Information_retrieval_query_language" class="mw-redirect" title="Information retrieval query language">information retrieval</a></li></ul></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="Database_security" title="Database security">Security</a></th><td class="navbox-list-with-group navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Database_activity_monitoring" title="Database activity monitoring">Activity monitoring</a></li>

<li><a href="Database_audit" title="Database audit">Audit</a></li>

<li><a href="Database_forensics" title="Database forensics">Forensics</a></li>

<li><a href="Negative_database" title="Negative database">Negative database</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="Database_design" title="Database design">Design</a></th><td class="navbox-list-with-group navbox-list navbox-even hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Entity%E2%80%93relationship_model" title="Entity–relationship model">Entities and relationships</a> (and <a href="Enhanced_entity%E2%80%93relationship_model" title="Enhanced entity–relationship model">Enhanced</a> notation)</li>

<li><a href="Database_normalization" title="Database normalization">Normalization</a></li>

<li><a href="Database_schema" title="Database schema">Schema</a></li>

<li><a href="Database_refactoring" title="Database refactoring">Refactoring</a></li>

<li><a href="Cardinality_(data_modeling)" title="Cardinality (data modeling)">Cardinality</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Programming</th><td class="navbox-list-with-group navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Database_abstraction_layer" title="Database abstraction layer">Abstraction layer</a></li>

</ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">Management</th><td class="navbox-list-with-group navbox-list navbox-even hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Database_virtualization" title="Database virtualization">Virtualization</a></li>

<li><a href="Database_tuning" title="Database tuning">Tuning</a>

<ul><li><a href="Database_caching" title="Database caching">caching</a></li></ul></li>

<li><a href="Data_migration#Database_migration" title="Data migration">Migration</a></li>

<li><a href="Database_preservation" title="Database preservation">Preservation</a></li>

<li><a href="Database_integrity" class="mw-redirect" title="Database integrity">Integrity</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%"><a href="Lists_of_databases" title="Lists of databases">Lists of</a></th><td class="navbox-list-with-group navbox-list navbox-odd hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="List_of_academic_databases_and_search_engines" title="List of academic databases and search engines">Academic</a></li>

<li><a href="List_of_biological_databases" title="List of biological databases">Biological</a></li>

<li><a href="List_of_biodiversity_databases" title="List of biodiversity databases">Biodiversity</a></li>

<li><a href="List_of_facial_expression_databases" title="List of facial expression databases">Facial expression</a></li>

<li><a href="List_of_online_databases" title="List of online databases">Online</a></li>

<li><a href="List_of_online_music_databases" title="List of online music databases">Online music</a></li>

<li><a href="List_of_online_real_estate_databases" title="List of online real estate databases">Online real estate</a></li></ul>

</div></td></tr><tr><th scope="row" class="navbox-group" style="width:1%">See also</th><td class="navbox-list-with-group navbox-list navbox-even hlist" style="width:100%;padding:0"><div style="padding:0 0.25em">

<ul><li><a href="Database-centric_architecture" title="Database-centric architecture">Database-centric architecture</a></li>

<li><a href="Intelligent_database" title="Intelligent database">Intelligent database</a></li>

<li><a href="Two-phase_locking" title="Two-phase locking">Two-phase locking</a></li>

<li><a href="Locks_with_ordered_sharing" title="Locks with ordered sharing">Locks with ordered sharing</a></li>

<li><a href="Database_publishing" title="Database publishing">Publishing</a></li>

<li><a href="Halloween_Problem" title="Halloween Problem">Halloween Problem</a></li>

<li><a href="Log_shipping" title="Log shipping">Log shipping</a></li></ul>

</div></td></tr><tr><td class="navbox-abovebelow hlist" colspan="2"><div><b><span class="noviewer" typeof="mw:File"><span title="WikiProject"></span></span> WikiProject</b> <b><span class="noviewer" typeof="mw:File"><span title="Category"></span></span> Category</b></div></td></tr></tbody></table></div></div><div><div class="zim-footer">

This article is issued from <a class="external text" title="Last edited on 2025-07-29" href="https://en.wikipedia.org/wiki/?title=Object%E2%80%93relational_mapping&oldid=1303147461">Wikipedia</a>. The text is available under <a class="external text" href="https://creativecommons.org/licenses/by-sa/4.0/deed.en">Creative Commons Attribution-Share Alike 4.0</a> unless otherwise noted. Additional terms may apply for the media files.

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