Question

Let’s say I have two Entity classes: SocialApp and SocialAppType

In SocialApp I have one Attribute: appURL and one Relationship: type.

In SocialAppType I have three Attributes: baseURL, name and favicon.

The destination of the SocialApp relationship type is a single record in SocialAppType.

As an example, for multiple Flickr accounts, there would be a number of SocialApp records, with each record holding a link to a person’s account. There would be one SocialAppType record for the "Flickr" type, that all SocialApp records would point to.

When I build an application with this schema, I get a warning that there is no inverse relationship between SocialAppType and SocialApp.

 /Users/username/Developer/objc/TestApp/TestApp.xcdatamodel:SocialApp.type: warning: SocialApp.type -- relationship does not have an inverse

Do I need an inverse, and why?

Answer 1

In practice, I haven’t had any data loss due to not having an inverse – at least that I am aware of. A quick Google suggests you should use them:

An inverse relationship doesn’t just
make things more tidy, it’s actually
used by Core Data to maintain data
integrity.

— Cocoa Dev Central

You should typically model
relationships in both directions, and
specify the inverse relationships
appropriately. Core Data uses this
information to ensure the consistency
of the object graph if a change is
made (see “Manipulating Relationships
and Object Graph Integrity”). For a
discussion of some of the reasons why
you might want to not model a
relationship in both directions, and
some of the problems that might arise
if you don’t, see “Unidirectional
Relationships.”

— Core Data Programming Guide