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In Core Data, most of the time relationships are modeled bidirectional. But the docs say in another place:
It typically only makes sense to model
a to-one relationship in one
direction.
Within Core Data you should always use a bi-directional relationship unless you have an extreme edge case. If you use one directional relationships then you are going to incur performance penalties within core data itself as well as have issues with referential integrity.
Unless you know specifically why you need a uni-directional relationship then you should always do a bi-directional relationship; the rule is that simple.
While Franci’s answer is interesting, I have to disagree with it. Even in the examples he provided you should have a bi-directional relationship. There are almost no situations where a uni-directional relationship is going to be a better fit.