I'm not sure. If we look in `ClassMetadataInfo::mapField()`, `inversedBy` and `mappedBy` are only checked for reference relationships. I believe PersistentCollection is used for embedded relationships as well, so I wouldn't expect either field in the mapping to be set. In that case, we might see an error in the `setOwner()` method added here.

Also, `ClassMetadataInfo::mapField()` has its own `isset()` checks around both fields, so that's not helpful in telling us if they're always set or not, even on a reference relationship. I looked in the Reference annotation classes and the XML and YML drivers, and it seems that these keys will always exist in the mapping array (after the driver does its processing), but they're prone to be `null` if unset. In that case, `backRefFieldName` would end up becoming null.