AdvancedDataMigration.md

Advanced Data Migration

Summary

You may find that in some cases you had additional relations or fields applied to a Block, or a field name has changed from your old block to a new element. This is best resolved using SilverStripe's DataExtension.

Example

MyBlock.php

<?php

namespace Foo\Bar;

class MyBlock extends Block
{
    private static $db = [
        'MyUniqueField' => 'Boolean',
    ];
}

MyElement.php

<?php

namespace Foo\Baz;

class MyElement extends BaseElement
{
    private static $db = [
        'MyNewUniqueField' => 'Boolean',
    ];
}

In this example MyUniqueField is now MyNewUniqueField. The migration tool isn't inherently aware of this change, however, we can use a DataExtension to handle this:

<?php

namespace Foo\Baz;

class MyDataExtension extends DataExtension
{
    private static $db = [
        'MyUniqueField' => 'Boolean',
    ];

    public function onBeforeWrite() {
        parent::onBeforeWrite();
    
        $this->owner->MyNewUniqueField = $this->owner->MyUniqueField;
    }
}

Applying the above DataExtension to MyElement will allow it to access the legacy field as it is re-implemented in the DataExtension's $db fields. We then us an onBeforeWrite() to move the value from the old field to the new field. After the migration is complete, this DataExtension could be removed if the legacy field is no longer needed, or at minimum, updating or removing the onBeforeWrite() so as to not overwrite any data after the migration is complete.