it's because with this PR a `Relationship` won't provide a field name, but require a field name.
Reasoning:
1 - Since models own fields (and their related datas), imo it make sense models need to decide on which field name a relationship must be binded.
2 - If field name generation is delegated to `Relationship`, model need to instanciate all relations to know on wich field name they have been associated, which is not compatible with the lazy loading.