Requirement: Via APEX dataloader upload attachments (standard object) and link it via ParentId to a custom object's record. This works easily when the SFDC ID is provided for the polymorphic ParentId on attachment object. This does not seem to work using an external ID.
But! There seems to be a way, as per SFDC documentation.
However, I can't get this to work by using the APEX dataloader.
Question: can anyone get this to work via dataloader (because that is the requirement)? If so, what's the trick? I can also imagine, this does not function via the dataloader, but it does work via a custom SOAP or REST API, or potentially using DML statements.
Any clarification would be much appreciated.