After hours of testing, I have the following assumption (conclusion) regarding creating a related record (if there is any mistake please point it out):
even it is a 1-to-1 situation, for one model ("Albums" in the example from the doc) to save, it has to be "belongsTo".
in the doc, it describes "belongsTo" as 1-to-many (only), perhaps it is a little bit misleading; and maybe it could be more clear if the doc explicitly tells which relationship definition is required to create related record?
