如何使用Swift从CoreData sqlite表中获取特定的行数据

Question

I am new to iOS development using Swift language.

I am using CoreData in my app as a database option.

I am using NSFetchRequest for fetching records from the table. I can retrieve all records from the table, but I can't retrieve a specific row from the same table. I have checked for the solutions online and other forums, but I am unable to get a proper solution for this.

I want to implement this in Swift only. I don't want to add the sqlite library or Bridging-wrapper (Objective - C) which will be my last option to implement this.

Any links or tutorials or suggestions will be helpful.

Answer 1

NSFetchRequest also support NSPredicate. With NSPredicate you can choose which exactly rows or row you need from Core Data.

More about NSPredicate - https://developer.apple.com/library/mac/documentation/Cocoa/Reference/Foundation/Classes/NSPredicate_Class/

Answer 2

You can fetch a desired row by executing a fetch request, casting it as an array, and simply using subscript to retrieve an index. Or you can narrow down your results using a unique id and a predicate and getting the object at index 0.

if let results = managedObjectContext?.executeFetchRequest(fetchRequest, error: nil) as? [SomeClass] {
    let someObject = results[someIndex]
}

Answer 3

As Pavel said, use NSPredicate. Here is a working example from my code, it might help you to get started ;) I added some comments below relevant lines

var req = NSFetchRequest(entityName: "YourDBTable") 
   // your db-table goes here
req.sortDescriptors = [NSSortDescriptor(key: "timeMillis", ascending: true)] 
   // if you want to sort the results
let start = NSNumber(longLong:int64StartValue) 
   // you need to convert your numbers to NSNumber
let end = NSNumber(longLong:int64EndValue)
let pred = NSPredicate(format:"timeMillis>=%@ AND timeMillis <=%@ AND foreignTableObject=%@",start,end, foreignTableObject)
    // if you have relationships, you can even add another NSManagedObject
    // from another table as filter (I am doing this with foreignTableObject here)
req.predicate = pred

var fetchedResults = managedContext?.executeFetchRequest(req,error: &error) as! [YourDBTable]? 
   // YourDBTable-class was created using the Xcode data model -> Editor -> Create NSManagedObject SubClass... - option
if let results = fetchedResults {
   // iterate through your results
}