Why do I get an error updating a custom field type from 2007 to 2010?

Question

I'm upgrading a publishing site from 2007 to 2010, the upgrade completes with errors in the upgrade log, all of which relate to a particular custom field type. An example of one the errors is here:

[powershell] [SPSiteWssSequence2] [ERROR] [07/11/2011 16:21:42]: 
Feature upgrade incomplete for Feature 'PublishingWeb' (Id: 'xxxxxxxxxxxxxxxxx') in Web 'http://xxxxtest01'. 
Exception: Field type YourCustomFieldType is not installed properly. 
Go to the list settings page to delete this field. 

The field type is being used in the site content so we can't go ahead and delete it (although it is tempting) - are there any known issues we could look into which affect the upgrading of field types?

Answer 1

I have fixed this issue by adding the following field to the to the fldtypes_MyCustomField.xml file:

<Field Name="CAMLRendering">TRUE</Field>

My thinking is that this prevents SharePoint from trying to update the field to to the new-fangled 2010 rendering and just uses the old 2007 CAML rendering instead.

I also found that updating other custom field types to use CAML Rendering also solved some other issues we were having with deploying content types. I'm not completely comfortable with this "solution" because I don't entirely understand it and I don't know what side-effects there may be. However, I can't deny that I now have a working site for the first time in quite a while.