Question

I’m not sure if I’m using the correct term here, but let’s say I have four entities in my model: Person, Place, Tag, and Photo. Any of the other three entities can have a Photo related to it. Sometimes a Person will have taken the photo and will attach it to a Tag, or a Place, or even another Person. What is the best way to approach polymorphic associations in CoreData?

Answer 1

I do not recommend entity inheritance for something as simple as this. For this design you have three relationships, each of the Person, Place and Tag have a relationship to Photo.Photo` has three inverse relationships. It does not need to be any more complicated than that.

Entity inheritance is a very delicate tool that can easily cause you many problems. Any entities that inherit from another will be flattened into a single table. If, as @morningstar suggested you created a Noun entity and have Person, Place and Tag inherit from it you will have ONE table in the SQLite file with indexes pointing to itself and other nastiness.

When to use entity inheritence?

That is a rather hard question to answer with a simple rule. However, I would say that a baseline would be to make sure that the resulting table has at least a 70% fill. For example, if you have an abstract with 6 attributes and two children with 2 attributes then that would probably be about an 80% fill rate and potentially ok.

In general, I do not use entity inheritence as it has little benefit and a great deal of risk to performance. The old rule works, don’t use it unless you know you need to use it.