You must set names for the relations. Have a look at the (just improved) section called Relation Names and multiple Relations of the documentation on relations. It comes with an example:
Property pictureIdProperty = user.addLongProperty("pictureId").getProperty();
Property thumbnailIdProperty = user.addLongProperty("thumbnailId").getProperty();
user.addToOne(picture, pictureIdProperty);
user.addToOne(picture, thumbnailIdProperty, "thumbnail");