When importing a dataset with "foreign key" relations, these may be specified with a natural key (e.g. "name" / "sku" / etc) - rather than the internal InvenTree ID - especially when importing from a different dataset.

So, add some options to lookup using other fields in the related model, rather than just "pk" value. "pk" is used as a priority, when a valid pk is provided.

For example when importing rows for a purchase order, supplier parts could be looked up by "sku" - this will be present when importing a purchase order dataset from an external supplier.