Answering myself, I found the answer there: https://groups.google.com/forum/?fromgroups#!topic/google-appengine/PyEdQs9UTBo
Supposing that the primary keys are in field "id", the __key__
import/export snippets in bulkloader.yaml should read:
- property: __key__
external_name: id
export_transform: transform.key_id_or_name_as_string
import_transform: transform.create_foreign_key('id', key_is_id=True)
