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<head><title>Referential Integrity</title>
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<h1>Referential Integrity</h1>
<p>db4o does not have a built-in referential integrity checking
mechanism. Luckily EventRegistry gives you access to all the necessary events
to implement it. You will just need to trigger validation on create, update or
delete and cancel the action if the integrity is going to be broken.</p>
<p>For example, if Car object is referencing Pilot and the
referenced object should exist, this can be ensured with the following handler
in deleting() event:</p>
<div class="codesnippet" MadCap:conditions="Primary.c#">
<pre class="prettyprint" xml:space="preserve">private static void ReferentialIntegrityCheck(object sender,
CancellableObjectEventArgs eventArguments)
{
Object toDelete = eventArguments.Object;
if (toDelete is Pilot)
{
IObjectContainer container = eventArguments.ObjectContainer();
IEnumerable<Car> cars = from Car c in container
where c.Pilot == toDelete
select c;
if (cars.Count() > 0)
{
eventArguments.Cancel();
}
}
}</pre>
<div class="codesnippet-meta">CallbackExamples.cs: Referential integrity
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<div class="codesnippet" MadCap:conditions="Primary.VB.NET">
<pre class="prettyprint lang-vb" MadCap:conditions="Primary.Online" xml:space="preserve">Private Shared Sub ReferentialIntegrityCheck(ByVal sender As Object, ByVal eventArguments As CancellableObjectEventArgs)
Dim toDelete As Object = eventArguments.Object
If TypeOf toDelete Is Pilot Then
Dim container As IObjectContainer = eventArguments.ObjectContainer()
Dim cars As IEnumerable(Of Car) = From c As Car In container _
Where c.Pilot Is toDelete
If cars.Count() > 0 Then
eventArguments.Cancel()
End If
End If
End Sub</pre>
<div class="codesnippet-meta">CallbackExamples.vb: Referential integrity
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<div class="codesnippet" MadCap:conditions="Primary.c#">
<pre class="prettyprint" xml:space="preserve">IEventRegistry events = EventRegistryFactory.ForObjectContainer(container);
events.Deleting += ReferentialIntegrityCheck;</pre>
<div class="codesnippet-meta">CallbackExamples.cs: Register handler
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<div class="codesnippet" MadCap:conditions="Primary.VB.NET">
<pre class="prettyprint lang-vb" MadCap:conditions="Primary.Online" xml:space="preserve">Dim events As IEventRegistry = EventRegistryFactory.ForObjectContainer(container)
AddHandler events.Deleting, AddressOf ReferentialIntegrityCheck</pre>
<div class="codesnippet-meta">CallbackExamples.vb: Register handler
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<p>You can also add handlers for creating() and updating()
events for a Car object to make sure that the pilot field is not null.
</p>
<p>Note, that in client/server mode deleting event is only raised on the server side, therefore the code above can't be used and will throw an exception. </p>
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