update key values of NSManagedObjects safely

Question

I'm trying to pass key value pairs and have the attributes updated through a loop.

func update(storableClass : NSManagedObject.Type, id: ID, fields: [String : Any]) throws {

    // retreive manaagedObject
    for (key, value) in fields {
        manaagedObject.setValue(value, forKey: key)
    }
}

Apparently the setValue(_:forKey:) will throw an exception if the key doesn't exist.

From what I've learned you're not to catch an Objective-C exception in Swift.

Is there any safe way to update core data properties through a dictionary?

I know I can have a function like below:

func update(storableClass : ManagedObject.Type, id: ID, closure: (ManagedObject) -> ()) throws {}

and then call it as such:

update(storableClass: UserEntity.self, id: "123123", closure: { userEntity in
    userEntity.name = "new name" 
})

I like to have both options...and any other safe/swifty option...

@rmaddy Correct me if I'm wrong. I think objective-c is related because `NSManagedObject` are based off of `NSObject` and the `setValue` function is from objective-c... — mfaani, Dec 10 '18 at 22:03
Every UI class is written in Objective-C too. Your question is about Swift and how to deal with mistakenly calling `setValue`, in Swift, with an unknown key. The better question for you is why are you calling `setValue` with an unknown key? That's a programming error. You should try to catch the exception, you should avoid causing it. — rmaddy, Dec 10 '18 at 22:11
Hmmm. You mean _That's a programming error_ vs. a **user** error or something out my control e.g. **server**? — mfaani, Dec 10 '18 at 22:20
the server isn't supposed to return a key that has no existence in your CD — Shehata Gamal, Dec 10 '18 at 22:21
Correct. There a difference between writing incorrect code and properly handling data. — rmaddy, Dec 10 '18 at 22:22
@Sh_Khan right. But I have no control over what server sends... — mfaani, Dec 10 '18 at 22:22
then loop for the keys before inserting to verify it's in your model or not — Shehata Gamal, Dec 10 '18 at 22:24
@rmaddy How do I catch programming errors? I would then needs some extensive manual or automated testing. My current NSManabgedObject has 15 fields and some other relationships... — mfaani, Dec 10 '18 at 22:25
@Sh_Khan how do I verify if it's among the keys? I'm not aware of NSObject's `keys` property... — mfaani, Dec 10 '18 at 22:26

score 4 · Accepted Answer · answered Dec 10 '18 at 22:59

I've had this exact problem. Here's what you do.

With Core Data, you can easily find out what property names are valid for the object. Get a reference to the managed object's entity, and then ask the entity for the properties. You'd use managedObject.entity.propertiesByName. The result is a dictionary where the keys are valid properties. The code would be something like

let properties = managedObject.entity.propertiesByName
for (key, value) in fields {
    if properties[key] != nil {
        managedObject.setValue(value, forKey: key)
    } else {
        print("Unknown key: \(key)")
    }
}

This is fine as long as value is the right type. If you start getting bogus data types in JSON, you have a more complex problem to solve.

For relationships, do the same thing with managedObject.entity.relationshpsByName.

FWIW there is also a type func called `entity()` which will have the same `propertiesByName` & `relationshpsByName`. Also a very good related read from Tom can be found [here](http://www.cimgf.com/2011/06/02/saving-json-to-core-data/) — mfaani, Dec 12 '18 at 22:08
Hi Tom, happy holidays. When you get a chance can you take a look [here](https://stackoverflow.com/questions/53891194/what-are-the-functional-differences-between-coredatas-codegen-manual-none-cr). I don't think it would require any coding, hopefully you can validate my observations off top of your head :) — mfaani, Dec 24 '18 at 17:31

update key values of NSManagedObjects safely

1 Answers1