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<h1>Migrating Between Databases</h1>
<p>Transparent activation and persistence functionality depends
on an association between an object and an object container, which is created
when an activator is bound to the object. Each object allows only one
activator. Typically this limitation won't show up, however there is a valid
use case for it: </p>
<p>1) suppose
you need to copy one or more objects from one object container to another;</p>
<p>2) you
will retrieve the object(s) from the first object container using any suitable
query syntax;</p>
<p>3) optionally
you can close the first object container;</p>
<p>4) you
will now save the object to the second object container.</p>
<p>If both object containers were
using transparent activation or persistence - the 4-th step will throw an
exception. Let's look at the case in more detail. Typical activatable class
contains an <code>activator</code> field. When transparent activation
functionality is used for the first time an object container activator will be
bound to the object:</p>
<p MadCap:conditions="Global.Primary:java" />
<p MadCap:conditions="Primary..NET,Primary.c#,Primary.All languages">
<pre class="prettyprint" xml:space="preserve">SensorPanelTA.cs: Bind
/**//*Bind the class to the specified object container, create the activator*/
public void Bind(IActivator activator)
{
if (_activator == activator)
{
return;
}
if (activator != null && null != _activator)
{
throw new System.InvalidOperationException();
}
_activator = activator;
}</pre>
</p>
<p MadCap:conditions="Global.Primary:cs" />
<p MadCap:conditions="Primary..NET,Primary.VB.NET,Primary.All languages">
<pre class="prettyprint lang-vb" xml:space="preserve">SensorPanelTA.vb: Bind
' Bind the class to the specified object container, create the activator
Public Sub Bind(ByVal activator As IActivator) Implements IActivatable.Bind
If _activator Is activator Then
Return
End If
If Not (activator Is Nothing Or _activator Is Nothing) Then
Throw New System.InvalidOperationException()
End If
_activator = activator
End Sub</pre>
</p>
<p MadCap:conditions="Global.Primary:vb" />
<p>If <code>bind</code> method will
be re-called with the same object container, activator parameter will always be
the same. However, if another object container tries to bind the object (in our
case with the <code>store</code> call) activator parameter will be different,
which will cause an exception. (Exception will be thrown even if the first
object container is already closed, as activator object still exists in the
memory.) This behaviour is illustrated with the following example
(<a href="../detailed_example.htm">SensorPanelTA</a> class from Transparent <span class="MCPopup"><a href="javascript:void(0);" class="MCPopupSpot" onclick="FMCPopup( event, this ); return false;" MadCap:src="../../../activation.htm">Activation<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> chapter is used):</p>
<p MadCap:conditions="Global.Primary:java" />
<p MadCap:conditions="Primary..NET,Primary.c#,Primary.All languages">
<pre class="prettyprint" xml:space="preserve">TAExample.cs: TestSwitchDatabases
private static void TestSwitchDatabases()
{
StoreSensorPanel();
IObjectContainer firstDb = Db4oFactory.OpenFile(ConfigureTA(), FirstDbName);
IObjectContainer secondDb = Db4oFactory.OpenFile(ConfigureTA(), SecondDbName);
try
{
IObjectSet result = firstDb.QueryByExample(new SensorPanelTA(1));
if (result.Count > 0)
{
SensorPanelTA sensor = (SensorPanelTA)result[0];
firstDb.Close();
// Migrating an object from the first database
// into a second database
secondDb.Store(sensor);
}
}
finally
{
firstDb.Close();
secondDb.Close();
}
}</pre>
</p>
<p MadCap:conditions="Global.Primary:cs" />
<p MadCap:conditions="Primary..NET,Primary.VB.NET,Primary.All languages">
<pre class="prettyprint lang-vb" xml:space="preserve">TAExample.vb: TestSwitchDatabases
Private Shared Sub TestSwitchDatabases()
StoreSensorPanel()
Dim firstDb As IObjectContainer = _
Db4oFactory.OpenFile(ConfigureTA(), FirstDbName)
Dim secondDb As IObjectContainer = _
Db4oFactory.OpenFile(ConfigureTA(), SecondDbName)
Try
Dim result As IObjectSet = _
firstDb.QueryByExample(New SensorPanelTA(1))
If result.Count > 0 Then
Dim sensor As SensorPanelTA = _
DirectCast(result(0), SensorPanelTA)
firstDb.Close()
' Migrating an object from the first database
' into a second database
secondDb.Store(sensor)
End If
Finally
firstDb.Close()
secondDb.Close()
End Try
End Sub</pre>
</p>
<p MadCap:conditions="Global.Primary:vb" />
<p>The solution to this problem is
simple: activator should be unbound from the object:</p>
<p MadCap:conditions="Global.Primary:java" />
<p MadCap:conditions="Primary..NET,Primary.c#,Primary.All languages">
<p>c#:</p>
<p><code>sensor.Bind(null);</code>
</p>
</p>
<p MadCap:conditions="Global.Primary:cs" />
<p MadCap:conditions="Primary..NET,Primary.VB.NET,Primary.All languages">
<p>VB:</p>
<p><code>sensor.Bind(Nothing)</code>
</p>
</p>
<p MadCap:conditions="Global.Primary:vb" />
<p>Note, that the object will quit
being activatable for the first object container. The following example shows
the described behaviour:</p>
<p MadCap:conditions="Global.Primary:java" />
<p MadCap:conditions="Primary..NET,Primary.c#,Primary.All languages">
<pre class="prettyprint" xml:space="preserve">TAExample.cs: TestSwitchDatabasesFixed
private static void TestSwitchDatabasesFixed()
{
StoreSensorPanel();
IObjectContainer firstDb = Db4oFactory.OpenFile(ConfigureTA(), FirstDbName);
IObjectContainer secondDb = Db4oFactory.OpenFile(ConfigureTA(), SecondDbName);
try
{
IObjectSet result = firstDb.QueryByExample(new SensorPanelTA(1));
if (result.Count > 0)
{
SensorPanelTA sensor = (SensorPanelTA)result[0];
// Unbind the object from the first database
sensor.Bind(null);
// Migrating the object into the second database
secondDb.Store(sensor);
System.Console.WriteLine("Retrieving previous query results from "
+ FirstDbName + ":");
SensorPanelTA next = sensor.Next;
while (next != null)
{
System.Console.WriteLine(next);
next = next.Next;
}
System.Console.WriteLine("Retrieving previous query results from "
+ FirstDbName + " with manual activation:");
firstDb.Activate(sensor, Int32.MaxValue);
next = sensor.Next;
while (next != null)
{
System.Console.WriteLine(next);
next = next.Next;
}
System.Console.WriteLine("Retrieving sensorPanel from " + SecondDbName + ":");
result = secondDb.QueryByExample(new SensorPanelTA(1));
next = sensor.Next;
while (next != null)
{
System.Console.WriteLine(next);
next = next.Next;
}
}
}
finally
{
firstDb.Close();
secondDb.Close();
}
}</pre>
</p>
<p MadCap:conditions="Global.Primary:cs" />
<p MadCap:conditions="Primary..NET,Primary.VB.NET,Primary.All languages">
<pre class="prettyprint lang-vb" xml:space="preserve">TAExample.vb: TestSwitchDatabasesFixed
Private Shared Sub TestSwitchDatabasesFixed()
StoreSensorPanel()
Dim firstDb As IObjectContainer = _
Db4oFactory.OpenFile(ConfigureTA(), FirstDbName)
Dim secondDb As IObjectContainer = _
Db4oFactory.OpenFile(ConfigureTA(), SecondDbName)
Try
Dim result As IObjectSet = _
firstDb.QueryByExample(New SensorPanelTA(1))
If result.Count > 0 Then
Dim sensor As SensorPanelTA = _
DirectCast(result(0), SensorPanelTA)
' Unbind the object from the first database
sensor.Bind(Nothing)
' Migrating the object into the second database
secondDb.Store(sensor)
System.Console.WriteLine( _
"Retrieving previous query results from " + FirstDbName + ":")
Dim [next] As SensorPanelTA = sensor.NextSensor
While [next] IsNot Nothing
System.Console.WriteLine([next])
[next] = [next].NextSensor
End While
System.Console.WriteLine( _
"Retrieving previous query results from " + FirstDbName + _
" with manual activation:")
firstDb.Activate(sensor, Int32.MaxValue)
[next] = sensor.NextSensor
While [next] IsNot Nothing
System.Console.WriteLine([next])
[next] = [next].NextSensor
End While
System.Console.WriteLine( _
"Retrieving sensorPanel from " + SecondDbName + ":")
result = secondDb.QueryByExample(New SensorPanelTA(1))
[next] = sensor.NextSensor
While [next] IsNot Nothing
System.Console.WriteLine([next])
[next] = [next].NextSensor
End While
End If
Finally
firstDb.Close()
secondDb.Close()
End Try
End Sub</pre>
</p>
<p MadCap:conditions="Global.Primary:vb" />
<p MadCap:conditions="Primary.Online">Download example code:</p>
<p MadCap:conditions="Primary.Online">
<MadCap:conditionalText MadCap:conditions="Primary..NET,Primary.VB.NET,Primary.All languages"><a href="tamigratevb.zip">VB.NET </a>
</MadCap:conditionalText>
<MadCap:conditionalText MadCap:conditions="Primary..NET,Primary.c#,Primary.All languages"><a href="tamigratecs.zip">c# </a>
</MadCap:conditionalText>
</p>
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