How to add "one-to" part of "one-to-many" to fetch results - swift

Question

I want to be able to add the player's data the "to-one" part of the many relationships. The fetch does some aggregation for me, but I would like to know what player it belongs to.
I have a CoreData model that looks like the following:
I have a fetch request that looks like the following:
func statsPerPlayer(player: Players, managedContext: NSManagedObjectContext) -> [String: Int] {
var resultsDic = [String: Int]()
let fetchRequest = NSFetchRequest<NSFetchRequestResult>(entityName: "Shifts")
let predicate = NSPredicate(format: "playersRelationship = %#", player)
fetchRequest.predicate = predicate
let nsExpressionForKeyPath = NSExpression(forKeyPath: "timeOnIce")
let nsExpressionForFunctionMin = NSExpression(forFunction: "min:", arguments: [nsExpressionForKeyPath])
let nsExpressionDescriptionMin = NSExpressionDescription()
nsExpressionDescriptionMin.expression = nsExpressionForFunctionMin
nsExpressionDescriptionMin.name = "minShift"
nsExpressionDescriptionMin.expressionResultType = .integer16AttributeType
let nsExpressionForFunctionMax = NSExpression(forFunction: "max:", arguments: [nsExpressionForKeyPath])
let nsExpressionDescriptionMax = NSExpressionDescription()
nsExpressionDescriptionMax.expression = nsExpressionForFunctionMax
nsExpressionDescriptionMax.name = "maxShift"
nsExpressionDescriptionMax.expressionResultType = .integer16AttributeType
let nsExpressionForFunctionSum = NSExpression(forFunction: "sum:", arguments: [nsExpressionForKeyPath])
let nsExpressionDescriptionSum = NSExpressionDescription()
nsExpressionDescriptionSum.expression = nsExpressionForFunctionSum
nsExpressionDescriptionSum.name = "sumShift"
nsExpressionDescriptionSum.expressionResultType = .integer16AttributeType
let nsExpressionForFunctionAvg = NSExpression(forFunction: "average:", arguments: [nsExpressionForKeyPath])
let nsExpressionDescriptionAvg = NSExpressionDescription()
nsExpressionDescriptionAvg.expression = nsExpressionForFunctionAvg
nsExpressionDescriptionAvg.name = "avgShift"
nsExpressionDescriptionAvg.expressionResultType = .integer16AttributeType
let nsExpressionForFunctionCount = NSExpression(forFunction: "count:", arguments: [nsExpressionForKeyPath])
let nsExpressionDescriptionCount = NSExpressionDescription()
nsExpressionDescriptionCount.expression = nsExpressionForFunctionCount
nsExpressionDescriptionCount.name = "countShift"
nsExpressionDescriptionCount.expressionResultType = .integer16AttributeType
fetchRequest.propertiesToFetch = [nsExpressionDescriptionMin, nsExpressionDescriptionMax, nsExpressionDescriptionSum, nsExpressionDescriptionAvg, nsExpressionDescriptionCount]
fetchRequest.resultType = .dictionaryResultType
do {
let fetchArray = try managedContext.fetch(fetchRequest)
print(fetchArray)
resultsDic = fetchArray.first as! [String : Int]
} catch let error as NSError {
print("\(self) -> \(#function): Could not fetch. \(error), \(error.userInfo)")
}
return resultsDic
} //statsPerPlayer
The results look great and something like this:
[{
avgShift = 39;
countShift = 4;
maxShift = 89;
minShift = 6;
sumShift = 157;
}]
However, I would like to include the player that this data is for. How do I add the "to-one" part of the one-to-many relationship in the results?
Thanks!!

Answer 1

In your code above, just add the relevant relationship name to the properties to fetch:
fetchRequest.propertiesToFetch = ["playersRelationship", nsExpressionDescriptionMin, nsExpressionDescriptionMax, nsExpressionDescriptionSum, nsExpressionDescriptionAvg, nsExpressionDescriptionCount]
The dictionaries that are returned will then include the key "playersRelationship" with value set to the NSManagedObjectID of the corresponding Players object. You can then use the context's object(with:) method to access the Players object itself.
Update
So after some testing, it turns out:
a) CoreData gets confused regarding the count aggregate function if you include the relationship in the propertiesToFetch. That leads to the Invalid keypath (request for aggregate operation on a toOne-only keypath error.
b) CoreData gets confused for all the other aggregate functions if you include the relationship in the propertiesToFetch. (It calculates the aggregate across every object, not just those matching the predicate.)
The solution to both problems is to add the relationship as a GROUP BY property. CoreData then calculates the aggregates correctly and also correctly recognises count as a valid operation. So, add the following line:
fetchRequest.propertiesToGroupBy = ["playersRelationship"]