Relationships are unidirectional, which means that the path
that leads from the source to the destination can’t be traveled
in the opposite direction—you can’t use a relationship to go
from the destination to the source. So, although you can use a toPhoto relationship
to find the photo for a particular person, you can’t use this
relationship in reverse to get a person’s name.
Unidirectionality is enforced by the way a relationship is resolved. Resolving a relationship means finding the correct destination record or records given a specific source record.
Bidirectional relationships—in which you can look up records in either direction—can be created by adding a separate return-trip, or inverse, relationship. But there is no concept of a single relationship that is bidirectional.