How to sum a double attribute from CoreData in swift

Question

I asked a similar question here: Getting the sum of an array of doubles in swift

but I still haven\'t gotten to a solution. Since the last question I changed my core

Answer 1

You can use reduce to add the double values of the fetched managed objects. In the "combining closure" you have to get the double value of the totalWorkTimeInHours attribute:

let fetchRequest = NSFetchRequest(entityName: "Log")

var error : NSError?
let results = managedContext.executeFetchRequest(fetchRequest, error: &error)
if let logs = results as? [Log] {
    let totalHoursWorkedSum = reduce(logs, 0.0) { $0 + $1.totalWorkTimeInHours.doubleValue }
    println(totalHoursWorkedSum)
} else {
    println("fetch failed: \(error?.localizedDescription)")
}

Alternatively you can use "Key-Value Coding" to sum the double values in the array of fetched objects. This is quite similar to Lyndseys's answer, only without an explicit loop:

let fetchRequest = NSFetchRequest(entityName: "Log")
var error : NSError?

if let results  = managedContext.executeFetchRequest(fetchRequest, error: &error) {
    let logs = results as NSArray
    let sum = logs.valueForKeyPath("@sum.totalWorkTimeInHours") as NSNumber
    let totalHoursWorkedSum = sum.doubleValue
    println(totalHoursWorkedSum)
} else {
    println("fetch failed: \(error?.localizedDescription)")
}

But there is a better way: You create an "expression description" for the sum of all totalWorkTimeInHours values:

let expressionDesc = NSExpressionDescription()
expressionDesc.name = "sumOftotalWorkTimeInHours"
expressionDesc.expression = NSExpression(forFunction: "sum:",
         arguments:[NSExpression(forKeyPath: "totalWorkTimeInHours")])
expressionDesc.expressionResultType = .DoubleAttributeType

and then a fetch request which fetches only this sum:

let fetchRequest = NSFetchRequest(entityName: "Log")
fetchRequest.propertiesToFetch = [expressionDesc]
fetchRequest.resultType = .DictionaryResultType

var error : NSError?
if let results  = managedContext.executeFetchRequest(fetchRequest, error: &error) {
    let dict = results[0] as [String:Double]
    let totalHoursWorkedSum = dict["sumOftotalWorkTimeInHours"]!
    println(totalHoursWorkedSum)
} else {
    println("fetch failed: \(error?.localizedDescription)")
}

The advantage is that the sum is calculated on the SQLite level, you don't have to fetch all the objects into memory.

A possible disadvantage is that this request only fetches the values that are stored in the persistent store, and ignores any unsaved changes in the mangaged object context.