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<h1>Foreign Keys</h1>
<p>Foreign keys issue is closely connected to the issue of
<a href="identity_and_primary_keys.html" class="wikiLink">object idenity</a>. </p>
<p>In RDBMS foreign keys are created by designating a special
column (or set of columns) in 2 different tables to hold the same information.
The data in a row of one table is related to the data in a row of another table
if the values in those columns match. Usually primary key is used on one side
of the equation (can be replaced by unique key). Foreign keys are also called
<a href="referential_integrity.html" class="wikiLink">referential constraints</a>.</p>
<p>In db4o there is no concept of a primary key, therefore no
foreign key as well. Object relationships are established through direct object
references, which are kept in object fields. This approach is perfectly straightforward
and intuitive. It considerably simplifies the navigation: instead of issuing
more and more queries to access child (referenced) objects, you can just get
them by the reference in the object field. This is definitely much more
productive and requires less coding.</p>
However, object references fulfill only part of
foreign key responsibilities: in addition to referencing another data entity,
foreign key ensures the referential integrity. The <a href="referential_integrity.html" class="wikiLink">following
chapter</a> discusses how referential integrity can be implemented in db4o.
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This revision (2) was last Modified 2007-06-02T20:23:35 by Tetyana.
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