无法使用谓词作为字符串从coreData获取？

Question

I am unable to fetch data using a string as a direct argument to NSPredicate. Is there any difference between these two,

//Working case
fetchRequest.predicate = NSPredicate(format: "role == %@","GK")

In the above case, I am able to fetch the data with the predicate.

//Not Working
predicateString = String(format:"role == %@","GK")
fetchRequest.predicate = NSPredicate(format: predicateString)

Here I am unable to fetch the data.

The only difference between the above two cases is that I'm using a String variable to define the predicate. What is wrong here?

Answer 1

When the %@ is replaced with a string variable when instantiating a NSPredicate the string variable is automatically surrounded by quotes so in your example the predicate becomes role == 'GK'

If you want to use String(format:...) you need to add the quotes yourself but in my opinion it's better to use NSPredicate(format:...) directly instead to avoid issues like this.

Answer 2

Format of NSPredicate is as following, where it expects a format, and format contains equaliser.

    NSPredicate(format: <String>, <args: CVarArg...>)

---

    let predicate1 = NSPredicate(format: "role == %@", "GK")

Here, if you will check the predicate1.predicateFormat , then you will get:

"role == \\"GK\\"" ,

which equilise role with string "GK" and return result in array .

But for,

    let predicateString = String(format:"role == %@","GK")
    let predicate2 = NSPredicate(format: predicateString)

You replaced the equaliser with simple string, which fails to return anything. You can check with predicate1.predicateFormat which will return:

"role == GK"

Here you can use "role == 'GK'" if you want to use string.

Hope it will help.