CODEWITHSUNDEEP
objective-ccore-datarelationship
user4951
user4951

Reputation: 33080

One Way Relationship in Core Data

Every restaurant must have Latitude and Longitude. However, every LatitudeLongitude object must not have "a restaurant"

So I have a one way relationship and generate a compiler warning.

What's the catch?

Should I use fetchRelationship instead? But what is fetchRelationship?

Why is it one way? Why does it have predicate? Why it is called fetch? I read the documentation and doesn't seem to get what it really is.

Upvotes: 1

Views: 1253

Answers (2)

Zoltán
Zoltán

Reputation: 1450

OK, super-old question, but here's how I'd be solving it now.

Not all relationships have to be filled. Keep your inverse (two-way) relationships but you don't necessarily have to link them.

+ (void)createRestaurantWithCompletion:(void (^)(BOOL, NSError *))completion {
    [MagicalRecord saveWithBlock:^(NSManagedObjectContext *localContext) {
        Restaurant *localRestaurant;
        LatitudeLongitude *localLatLng1;
        LatitudeLongitude *localLatLng2;

        restaurant = [Restaurant MR_createInContext:localContext];

        localLatLng1 = [LatitudeLongitude MR_createInContext:localContext];
        localLatLng2 = [LatitudeLongitude MR_createInContext:localContext];

        restaurant.latLng = localLatLng1;
    }                 completion:completion];
}

By the time completion is called, both localLatLng1 and localLatLng2 exist, one is linked to a restaurant, one isn't. Of course this method makes no sense in this form, it's just to prove the point that you can create objects without having to satisfy their relationships.

Z.

Upvotes: 1

David
David

Reputation: 7303

You should use fetched properties (which are an collection of managed objects satisfying a specified fetch predicate) to represent one-way relationships. More information here.

Update

You should probably use attributes in your case, otherwise you would add a fetched property to the Restaurant entity, set it's destination entity to LatitudeLongitude and store lattitude and longitude key-value pairs in its userInfo dictionary. Its fetch predicate would look like this:

($FETCH_SOURCE.longitude LIKE [c] $FETCHED_PROPERTY.userInfo.longitude) AND ($FETCH_SOURCE.latitude LIKE [c] $FETCHED_PROPERTY.userInfo.latitude)

Upvotes: 2

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