Lets say we have content type called MyContentType that has a field category with a keyword index. Suppose we then change our minds and want to use Plone's internal subject (aka Keyword) field for this purpose instead. If there are existing content types in your site, doing this manually would be a pain. Luckily, it's quite simple to write a quick python script to migrate the data from one field to another.

Even more luckily, there is now a component out there to help you called contentmigration!

The script method below may be all you need, but contentmigration, which leverages ATContentTypes' migration framework, used internally in Plone, is very flexible and allows you to make changes like the one below with less code. The tool is described in the context of other migrations in the RichDocument tutorial - see the last section of this page in particular.

If you would rather do it manually with a script, the original steps are below:

1. Backup your database
2. Create a script in you custom skin folder copyKeywords.py
3. Paste in the script below
4. In the catalog search put the name of you contenttype where it says MyContentType. This parameter ensures that the script only searches for your specific content type.
5. In this case, we are using getCategory() as the accessor for the old value, and setSubject() as the mutator for the new value. You may have different fields, and you may need to manipulate the value before writing it to the new field.
6. To run the script, click on the test tab in the zmi

Note: If an exception was raised, the transaction will be rolled back, and no data will be changed. If the script executes successfully, you can use the "Undo" tab at the root of your Plone site in the ZMI to undo the running of the script, if you change your mind

the script:

    request = container.REQUEST
    RESPONSE =  request.RESPONSE

    brains = context.portal_catalog.searchResults( REQUEST=request,
                                                   Type = 'myContent')

    for brain in brains:
        obj = brain.getObject()
        keywords = obj.getCategory()
        obj.setSubject(keywords)
        print "Updated", obj.absolute_url()

    return printed

There are many variations on this script. You may for example perform calculations or other modifications before saving the new value, or save the value to a different content object altogether. Remember that you are modifying your database, so be careful, don't forget the backup, and use the Undo tab if things go wrong!