Is it good practice to model to-one relationships in only one direction? Or must they be modeled in two directions?

Question

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.

Answer 1

The answer is determined by the referential integrity requirements you want to enforce. If updating or removing the object on either side affects the object on the other side of the relationship, you need two-way. However, if updating/removing the object on one side does not affect the object on the other, then a one way is a better model.

Take for example a parent-children model with a 0..n : 1 cardinality (I prefer the 1 : 0..n representation, but for the sake of argument let's reverse it). Adding a new child, updating an existing child or deleting a child has no effect on the parent object, so there's no need for the parent to know explicitly about all the children (except when it comes time to pay college tuition). However, removing the parent has an adverse effect on the children objects, as they need to be deleted or re-parented, otherwise are orphaned and in an invalid state. Thus, it's better to model it as a one-way relationship. Another example is inventory - parts catalog relationship, again with 0..n : 1 cardinality.

Answer 2

It's a matter of ownership: usually it doesn't make sense to have a bidirectional relationship because an entity conceptually owns the other one.

Think about some examples. If you have a structure in which you have users and an user can have a simple bank account associated with him. If you make the relation bidirectional you mean that an user owns an account but also an account owns an user.

This will make sense because you don't want to delete an user whenever you delete his account. That's why usually you don't need to have it bidirectional: because it's an additional constraint that is not needed since most of the time you will have an entity that has the other but not vice-versa.

Answer 3

I think you read the whole document about relations you referenced in your question.

The document also describes all disadvantages of using unidirectional relations, and that only under very rare circumstances it makes sense to create unidirectional relations.

As a general rule i would strongly recommend creating bidirectional relations, except you are knowing exactly why not to do so.

Answer 4

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.