<html>
<head>
<META http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>Unique Constraints</title>
<link rel="stylesheet" type="text/css" href="../../style.css">
</head>
<body>
<div class="CommonContent">
<div class="CommonContentArea">
<h1>Unique Constraints</h1><p>Unique Constraints feature was first introduced in db4o 6.2.</p>
<p>Unique Constraints allow a user to define a field
to be unique across all the objects of a particular Class stored to db4o. This
means that you cannot save an object where a previously committed object
has the same field value for fields marked as unique.<br>
<br>
A Unique Constraint is checked at commit-time and a constraint violation will
cause a UniqueFieldValueConstraintViolationException to be thrown. This
functionality is based on <a href="callbacks/commit-time_callbacks.html" class="wikiLink">Commit-Time Callbacks</a> feature. </p>
Multiple constraints can be defined on the
same class if required.
<p><div class="childTopicList">More Reading:<ul>
<li><p><a href="unique_constraints/how_to_use_unique_constraints.html" class="wikiLink">How To Use Unique Constraints</a></p></li>
<li><p><a href="unique_constraints/unique_constraints_example.html" class="wikiLink">Unique Constraints Example</a></p></li>
</ul></div>
</p></div>
</div>
<div id="footer">
This revision (3) was last Modified 2007-05-07T10:04:49 by Tetyana.
</div>
</body>
</html>
