Core Data: inverse relationship for two relationships with same type

Question

In my app Core Data model I have Sheet and Text entities. Sheet entity can have two Text's: privacyNotes and termsOfUse.

Both of Text type. So in XCode data modeler I create to-one relationships called "privacyNotes" and "termsOfUse" in Sheet with Text destination. Next goes to-one relationship "sheet" in Text. Then I select that Text.sheet relationship as inverse for Sheet.privacyNotes. So far so good. But when I set same Text.sheet relationship as inverse for Sheet.termOfUse XCode deletes this relationship as inverse Sheet.privacyNotes!

I understand that relationships in DB can be not so simple compared to Objective-C objects relationships, but I really don't get why SQLite or (CoreData) can't reuse one relationship as inverse for FEW other relationships?

Answer 1

A little peek under the abstraction hood might be enlightening*: a relation can only be the inverse for exactly one other relation because, in the backing store, they're represented by the same data. If a Text and a Sheet can have a certain relationship, Core Data does what a good human data modeler would do and stores that relationship as succinctly as possible. The relation properties of the entity objects are just ways of looking at that relationship.

To get the effect of what you're going for: go ahead and give Sheet properties for privacyNote and termsOfUse; but give Text properties like sheetIAmTermsFor and sheetIAmPrivacyNoteFor, and set them as inverses appropriately. Then in the Text class, add a synthetic property along these lines:

// in interface
@property (nonatomic, readonly) Sheet *sheet;
// in impl
-(Sheet *)sheet
{
  if ([self sheetIAmTermsFor])
    return [self sheetIAmTermsFor];
  else
    return [self sheetIAmPrivacyNoteFor];
}

If you want to write a setter too, you'll have to decide which role that setter should bestow on the Text (which Core Data can't figure out for you, another reason a property can't be the inverse of two different properties.)

If you need to enforce a constraint that a Text can only ever be a "privacyNote" or a "terms" but never both, override the setters for sheetIAmTermsFor and sheetIAmPrivacyNoteFor, following Apple's pattern in the docs, and have each null the other property when set.

(* Apple regards the SQLite databases Core Data generates as private to their implementation, but inspecting their schemas can be very educational. Just don't be tempted to write shipping code that goes behind CD's back to poke at the db directly.)

Answer 2

You are far better off having a one to many relationship between Sheet and Text with a validation limit of 2. Then you should have a type property in the text which declares it as either a privacyNotes or termsOfUse. From there you can add convenience methods to your Sheet subclass that allows you to retrieve either one.