Through consuming the Salesforce.com Partner WSDL, I'm working on synchronization between an external system and Salesforce.com. As you would probably expect, the objects that don't exist by default in Salesforce.com are being created as custom objects via the GUI. While I've had no problem porting over basic fields, I've had some problems with the relationships.
Some of the external system's relationships are defined by foreign keys which are polymorphic (as in, they can store the identifier for two or more different types). While I see that Salesforce.com makes references in their documentation in how to deal with polymorphic foreign keys in SOQL, I haven't found a way to actually create new polymorphic foreign key fields.
Is there any way to create new polymorphic foreign key fields in Salesforce.com, or do you have to create individual foreign key fields for each type that could potentially be referenced?