Variable Substitution with FetchRequests stored in

问题:

I've always created my NSFetchRequests entirely in-code. Now I'm looking at the Xcode GUI for building a fetch request and storing it in the model.

I'm following an example from the Xcode Documentation. I added a Fetch Request to my model, and the predicate that has been created through the Modelling GUI is:

 firstName LIKE[c] "*SUBSTRING*"

I then retrieve that request with these two lines:

NSDictionary *substituionDictionary = [NSDictionary dictionaryWithObject:@"woody" forKey:@"SUBSTRING"];

NSFetchRequest *fetchRequest = [mom fetchRequestFromTemplateWithName:@"firstNameContains" substitutionVariables:substituionDictionary];

An NSLog of the resulting NSFetchRequest outputs this:

(entity: Customer; predicate: (firstName LIKE[c] "*SUBSTRING*"); sortDescriptors: (null); limit: 0)

.. which indicates that the variable is not being substituted prior to the return of the stored FetchRequest.

So, how does one specify that text entered in the Xcode Data Modelling Fetch Request Predicate Builder GUI is intended to be substituted at runtime by NSFetchRequest:fetchRequestFromTemplateWithName:substitutionVariables: ?

Thank you!

Woody

回答1:

You need to right-click on the row of the fetch request predicate editor containing the intended variable and select "VARIABLE" from the popup. Where you right-click is sometimes picky in the Xcode editor, so I tend to click just to the left of the +/- buttons.

回答2:

Here is the example of substitution Variables.

First create the fetchRequest Template in the Fetch Requests Section.

Then write the code for fetch employee by firstName.

   func employeeByFirstName(fName: String) -> [Employee]{

    let viewContext = self.persistentContainer.viewContext

    var substitutionVariables: [String: Any] = [String : Any]()
    substitutionVariables["FIRSTNAME"] = fName
    let fetchRequest = fetchRequestBy(name: "fetchByFirstName", variables: substitutionVariables) as! NSFetchRequest<Employee>

    do {
        let objects = try viewContext.fetch(fetchRequest)
        return objects
    } catch  {
        let nserror = error as NSError
        fatalError("Unresolved error \(nserror), \(nserror.userInfo)")
    }
}


   func fetchRequestBy(name: String, variables: [String : Any]) -> NSFetchRequest<NSFetchRequestResult>? {
      let model = self.persistentContainer.managedObjectModel
      let fetchRequest = model.fetchRequestFromTemplate(withName: name, substitutionVariables: variables)
      return fetchRequest
   }