Swift - Get one-to-many relationship

Question

let's imagine that we have 2 entities:

-People (name, age, ..)

-House (color)

we recorded the data several times with house.addToPeople (newPeople) for each house

we want to get all the people of the house colored blue

how do we fetch this?

I tried this code but it gets all the people

        let appD = UIApplication.shared.delegate as! AppDelegate
            let context = appD.persistentContainer.viewContext
        
        
        let peopleFetch = NSFetchRequest<NSFetchRequestResult>(entityName: "People")

        let houseFetch = NSFetchRequest<NSFetchRequestResult>(entityName: "House")               
        houseFetch.fetchLimit = 1       
        houseFetch.predicate = NSPredicate(format: "color = %@", "blue")
    
        ...
        
        let res = try? context.fetch(peopleFetch)
        let resultData = res as! [People]

how to do this ?

Execute the fetch for the house instead (houseFetch), the other one is irrelevant, and then access the people form your house object using the `people` attribute — Joakim Danielson, Dec 20 '20 at 21:52

Federica Benacquista · Accepted Answer · 2021-01-12T12:28:50.513

Try this function. What it does is fetching all of the People and creating an array with all of the results.

func getAllItems() -> [People]? {
        let request = NSFetchRequest<NSFetchRequestResult>(entityName: "People")
        request.returnsObjectsAsFaults = false
        
        do {
            let result: NSArray = try context.fetch(request) as NSArray
            return (result as? [People])!
        } catch let error {
            print("Errore recupero informazioni dal context \n \(error)")
        }
        return nil
    }

If you want to perform your search following certain criteria such as a color, use the following code after request:

//Here i'm searching by index, if you need guidance for your case don't hesitate asking
 request.predicate = NSPredicate(format: "index = %d", currentItem.index)

Edit: actually the code above is just to get all of the people, if you want to base your search on the houses do the following:

  func retrieve()-> [People]{
//Fetch all of the houses with a certain name
        let houseRequest = NSFetchRequest<NSFetchRequestResult>(entityName: "House")
        houseRequest.predicate = NSPredicate(format: "name = %@", "newName") //This seearch is by name
        houseRequest.returnsObjectsAsFaults = false
        do {
//put the fetched items in an array
            let result: NSArray = try context.fetch(houseRequest) as NSArray
            let houses = result as? [Houses]
//Get the people from the previous array
            let people: [People]? = (houses.people!.allObjects as! [People])
            return people
        } catch let error {
            print("Errore recupero informazioni dal context \n \((error))")
        }
            return nil
        
    }

score 0 · Answer 2 · answered Dec 21 '20 at 21:03

0

Thank you for your answer !

In this example "houses" is an array, so we have to add an index ➔ houses[0].people!.AllObjects

And thank you very much for your explanation.

answered Dec 21 '20 at 21:03

Swift - Get one-to-many relationship

2 Answers2