CODEWITHSUNDEEP
iphoneobjective-ccore-data
dantastic
dantastic

Reputation: 134

fetchedresultscontroller with two entities - predicate to target each entity?

My iPhone app has a summary page (UITableView) where I would like to show a quick overview and therefore I need to get info from several entities. It was suggested to create an abstract parent entity and have my two entities as children of this one. This do allow me to fetch the two entities using the one fetchedresultscontroller.

This works but I find that I need to filter the return a small bit. Because of the 'hack' above these entities have nothing in common so I need completely separate predicates.so from EntityA I would need "color = blue" and from EntityB "length >= 10". Because the entity I'm actually querying have none of these this doesn't work at all.

Is there a way to do this or what's the best approach here?

Upvotes: 0

Views: 652

Answers (2)

TechZen
TechZen

Reputation: 64428

Niether the UITableView or the NSFetchedResultsController is designed to deal with more than one entity at a time. Moreover, it seldom makes any sense to try to do so. If you find yourself in such a situation, you probably need to rethink your data model design.

If entities are logically associated with each other then they should be linked by a relationship. If data from any two objects is to be displayed in the same tableviewCell without being gibberish then they must have some logical association and therefore should be linked by a relationship of some kind. To display in the table, you fetch one side of the relationship and then walk the relationship/s to find the other related objects.

If the logical association is strong, it should be defined as a formal relationship in the data model e.g.:

EntityA{
    //... some attributes
    b<-->B.a
}

EntityB{
    //...some attributes
    a<-->A.b
}

However, if the relationship is weak or transient, then you should use a fetched property to relate them. The fetched property dynamically searches for other entities based on a preprogrammed fetch. 

EntityA{
    creationDate:date
    someBs--(creationDate=$FETCH_SOURCE.creationDate)-->B
}

EntityB{
    creationDate:date
}

A key concept here is that Core Data is providing the entire model layer of your Model-View-Controller design. It is not just a dumb database but an active object graph that models both the data itself and its behavior. Once you have a properly designed data model, problems with the controllers and views resolve themselves automatically.

Upvotes: 1

Hanuman
Hanuman

Reputation: 642

If i understand correctly, you can use notifications and send a dictionary of required information to the UITableView view controller class.

Upvotes: 0

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