Core data parent and child entity - ios

Question

I have a small problem in Parent and child entity in core data model. My core data model has a Person entity which can have many googleplus account or twitter account linked to it. Since these two entities; GooglePlus and Twitter are somehow similar with the properties they have, I thought of creating an abstract entity called SocialConnection. Now, my Person entity has to-many relationship to SocialConnection entity which inturn is parent to both GooglePlus and Twitter.
Person <----->> SocialIdentifier ---- Child ---- GooglePlus
|
|
Child
|
|
Twitter
My graphical model is shown in the figure below.
Now, I have a situation here. Person has a property called socialConnections which will fetch all the connections regardless of if it is Twitter object or GooglePlus object. But, I would like to fetch just twitter sometime and GooglePlus sometimes. How would I do it.
I think one idea would be to have a backward relationship from SocialIdentifier pointing to the Person entity and let Person create forward relationship with each of the child.
It would be represented as;
Person <----------- SocialIdentifier -------- Child --------GooglePlus----->> Person
|
|
Child
|
|
Person <<------- Twitter
Graphical representation;
But, even with this representation, I am facing some problem. I could get googles and twitters from Person model but it has no reference to the SocialConnections ie. all the connections.
And, I feel this is redundant as the first one simplifies the design a lot. I am sure a little tweak to the first one would yield the desired one but I could not figure it out.
How could I fetch twitters or googles directly from the first model or is it possible to fetch all the connections from Person model without using predicate ofcourse. I am sure there are some pretty neat solution to this.
I would like to thank you for your help and suggestions beforehand.

Answer 1

I wouldn't set up new relationships for this. There are a couple of different approaches you could use.
Fetched property
Give Person a new fetched property named twitters. Set the destination entity to Twitter and the fetch predicate to person==$FETCH_SOURCE. Then just ask for the value of twitters when you need it. (Yeah, you said no predicate for some reason, but honestly, it's the right tool for the job here).
The advantage here is that you don't have to maintain a new relationship. If you add a new Twitter account to socialConnections, it automatically becomes part of twitters.
Filtering the set
If there won't be a large number of socialConnections then it might be easier to just filter it in code instead of getting Core Data to do it. Get every socialConnections object and filter the set, something like:
NSEntityDescription *twitterEntity = [NSEntityDescription entityForName:#"Twitter" inManagedObjectContext:[self managedObjectContext]];
NSSet *twitters = [[person valueForKey:#"socialConnections"] objectsPassingTest:^BOOL(id obj, BOOL *stop) {
return [[obj entity] isKindOfEntity:twitterEntity];
}];
That runs through every social connection and gets just the Twitter accounts.