I have an InfoPath form that has a lookup field to a different List. I wanted to set Metadata navigation on the InfoPath form list so that the Lookup field would show up in the Available Heirarchy fields, but it doesn't show up there. In fact unless I have a hard coded drop down field I can't get the Metadata navigation to work. I don't want to hard code the list of field values, I want it to rely on the external list. Is there any hope for me?
You need to create a choice column on the list that contains the same values as your lookup list. Then create a workflow that runs "On Edit" (if this is a document library) to copy the value of the lookup to the choice column. Then you can add this field to the metadata navigation.
Note: if you run the workflow "on create" the user will not be able to update any meta data once after they upload a file, the workflow will have the item locked.
The other option is to use a Managed Metadata Field instead of a lookup.