We are Upserting a custom object A using API calls from .Net code. In the Upsert call, we have been specifying External ID of the custom object A. Now, A has a lookup relationship with custom object B. We are thinking if A can be upserted and linked (with B) in one API call. For example, when we upsert using Data Loader, there is a way to specify lookup relationship in .sdl file.
We are searching for a similar way of doing this in our .Net code to save number of API calls to link B to A. In the Apex API guide, there are examples of Upserting objects with its own external IDs but not with related (lookup, Parent objects) objects by specifying their External IDs. Please share if anyone is having any idea about it.