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<h1><a name="kanchor82"></a>Autoincrement</h1>
<p>db4o does not deliver a field auto increment feature, which
is common in <span class="MCTextPopup"><a href="javascript:void(0);" class="MCTextPopupSpot" onclick="FMCTextPopup( event, this ); return false;">RDBMS<img style="border: none;margin-left: 5px;" src="../../../SkinSupport/ExpandingClosed.gif" MadCap:altsrc="../../../SkinSupport/ExpandingOpen.gif" class="MCExpandingIcon" onload="if ( typeof( FMCPreloadImage ) == 'function' ) { FMCPreloadImage( '../../../SkinSupport/ExpandingOpen.gif' ); }" /></a><span class="MCTextPopupBody" style="display: none; ">Relational Database Management System</span></span>. Normally you don't need any additional ids, since db4o manages objects by object-identity. However cases where you have <a href="../../../platform_specific_issues/disconnected_objects.htm">disconnected objects</a>, you need additional ids. One of then possibilities it to use auto incremented ids.</p>
<p> If your application logic requires this feature you can
implement it using external callbacks. One of the possible solutions is
presented below. Note that this example only works in embedded-mode.</p>
<p>This example assumes that all object which need an auto incremented id are subclasses of the IDHolder-class. This class contains the auto-incremented id.</p>
<div class="codesnippet" MadCap:conditions="Primary.c#">
<pre class="prettyprint" xml:space="preserve">private int id;
public int Id
{
get { return id; }
set { id = value; }
}
</pre>
<div class="codesnippet-meta">IDHolder.cs: id holder
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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 m_id As Integer
Public Property Id() As Integer
Get
Return m_id
End Get
Set(ByVal value As Integer)
m_id = value
End Set
End Property
</pre>
<div class="codesnippet-meta">IDHolder.vb: id holder
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<p>First create a class which keeps the state of the auto-increment numbers. For example a map which keeps the latest auto incremented id for each class.</p>
<div class="codesnippet" MadCap:conditions="Primary.c#">
<pre class="prettyprint" xml:space="preserve">private class PersistedAutoIncrements
{
private readonly IDictionary<Type, int> currentHighestIds = new Dictionary<Type, int>();
public int NextNumber(Type forClass)
{
int number;
if (!currentHighestIds.TryGetValue(forClass, out number))
{
number = 0;
}
number += 1;
currentHighestIds[forClass] = number;
return number;
}
}
</pre>
<div class="codesnippet-meta">AutoIncrement.cs: persistent auto increment
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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 Class PersistedAutoIncrements
Private ReadOnly currentHighestIds As IDictionary(Of Type, Integer) _
= New Dictionary(Of Type, Integer)()
Public Function NextNumber(ByVal forClass As Type) As Integer
Dim number As Integer
If Not currentHighestIds.TryGetValue(forClass, number) Then
number = 0
End If
number += 1
currentHighestIds(forClass) = number
Return number
End Function
End Class
</pre>
<div class="codesnippet-meta">AutoIncrement.vb: persistent auto increment
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<p>Then create two methods, which are called later. One which returns the next auto-incremented id for a certain class. Another which stores the current state of the auto-increments.</p>
<div class="codesnippet" MadCap:conditions="Primary.c#">
<pre class="prettyprint" xml:space="preserve">public int GetNextID(Type forClass)
{
lock (dataLock)
{
PersistedAutoIncrements incrementState = EnsureLoadedIncrements();
return incrementState.NextNumber(forClass);
}
}
public void StoreState()
{
lock (dataLock)
{
if (null != state)
{
container.Ext().Store(state,2);
}
}
}
</pre>
<div class="codesnippet-meta">AutoIncrement.cs: getting the next id and storing state
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<div class="codesnippet" MadCap:conditions="Primary.VB.NET">
<pre class="prettyprint lang-vb" MadCap:conditions="Primary.Online" xml:space="preserve">Public Function GetNextID(ByVal forClass As Type) As Integer
SyncLock dataLock
Dim incrementState As PersistedAutoIncrements = EnsureLoadedIncrements()
Return incrementState.NextNumber(forClass)
End SyncLock
End Function
Public Sub StoreState()
SyncLock dataLock
If state IsNot Nothing Then
container.Ext().Store(state, 2)
End If
End SyncLock
End Sub
</pre>
<div class="codesnippet-meta">AutoIncrement.vb: getting the next id and storing state
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<p>The last part is to ensure that the existing auto-increments are loaded from the database. Or if not existing a new instance is created.</p>
<div class="codesnippet" MadCap:conditions="Primary.c#">
<pre class="prettyprint" xml:space="preserve">private PersistedAutoIncrements EnsureLoadedIncrements()
{
if (null == state)
{
state = LoadOrCreateState();
}
return state;
}
private PersistedAutoIncrements LoadOrCreateState()
{
IList<PersistedAutoIncrements> existingState = container.Query<PersistedAutoIncrements>();
if (0 == existingState.Count)
{
return new PersistedAutoIncrements();
}
else if (1 == existingState.Count)
{
return existingState[0];
}
else
{
throw new InvalidOperationException("Cannot have more than one state stored in database");
}
}
</pre>
<div class="codesnippet-meta">AutoIncrement.cs: load the state from the database
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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 Function EnsureLoadedIncrements() As PersistedAutoIncrements
If state Is Nothing Then
state = LoadOrCreateState()
End If
Return state
End Function
Private Function LoadOrCreateState() As PersistedAutoIncrements
Dim existingState As IList(Of PersistedAutoIncrements) = container.Query(Of PersistedAutoIncrements)()
If 0 = existingState.Count Then
Return New PersistedAutoIncrements()
ElseIf 1 = existingState.Count Then
Return existingState(0)
Else
Throw New InvalidOperationException("Cannot have more than one state stored in database")
End If
End Function
</pre>
<div class="codesnippet-meta">AutoIncrement.vb: load the state from the database
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<p>Now it's time to use the callbacks. Every time when a new object is created, assign a new id. For this the creating-event is perfect. When commiting also make the auto increment-state persistent, to ensure that no id is used twice.</p>
<div class="codesnippet" MadCap:conditions="Primary.c#">
<pre class="prettyprint" xml:space="preserve">AutoIncrement increment = new AutoIncrement(container);
IEventRegistry eventRegistry = EventRegistryFactory.ForObjectContainer(container);
eventRegistry.Creating+=
delegate(object sender, CancellableObjectEventArgs args)
{
if (args.Object is IDHolder)
{
IDHolder idHolder = (IDHolder)args.Object;
idHolder.Id = increment.GetNextID(idHolder.GetType());
}
};
eventRegistry.Committing +=
delegate(object sender, CommitEventArgs args)
{
increment.StoreState();
};</pre>
<div class="codesnippet-meta">AutoIncrementExample.cs: use events to assign the ids
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</div>
<div class="codesnippet" MadCap:conditions="Primary.VB.NET">
<pre class="prettyprint lang-vb" MadCap:conditions="Primary.Online" xml:space="preserve">Dim increment As New AutoIncrement(container)
Dim eventRegistry As IEventRegistry = EventRegistryFactory.ForObjectContainer(container)
AddHandler eventRegistry.Creating, AddressOf increment.HandleCreating
AddHandler eventRegistry.Committing, AddressOf increment.HandleCommiting</pre>
<div class="codesnippet-meta">AutoIncrementExample.vb: use events to assign the ids
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<p>Last, don't forget to index the id-field. Otherwise looks-ups will be slow.</p>
<div class="codesnippet" MadCap:conditions="Primary.c#">
<pre class="prettyprint" xml:space="preserve">configuration.Common.ObjectClass(typeof (IDHolder)).ObjectField("id").Indexed(true);</pre>
<div class="codesnippet-meta">AutoIncrementExample.cs: index the id-field
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<div class="codesnippet" MadCap:conditions="Primary.VB.NET">
<pre class="prettyprint lang-vb" MadCap:conditions="Primary.Online" xml:space="preserve">configuration.Common.ObjectClass(GetType(IDHolder)).ObjectField("id").Indexed(True)</pre>
<div class="codesnippet-meta">AutoIncrementExample.vb: index the id-field
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