NSFetchRequest: Query for distinct values and count other properties values - core-data

Question

I try to set up an NSFetchRequest for an entity Location with properties like countryand city:
country | city
————————————————
Germany | Berlin
USA | San Francisco
USA | New York
Germany | Munich
Germany | Munich
USA | San Francisco
Germany | Stuttgart
The NSFetchRequest should return the country (or a Location object with the appropriate country) and the number of cities.
[
{ country: 'Germany', cityCount: 3 },
{ country: 'USA', cityCount: 2 }
]
I know that I can just fetch all entries and 'count it myself' but I am interested in how to set up an appropriate fetch request (or if it's possible to do so) and would like to see how you would do it! :)

Answer 1

The correct answer to this question to refactor the data model in order to avoid redundancy.
The country strings are repeated unnecessarily in the table. Additionally, you make a simple query gratuitously complicated. The model should reflect your data, and writing out "USA" for every American city is just not smart or efficient.
Your data model should look like this
Country <----->> City
Now you can just fetch all countries and get the cities with cities.count.

Answer 2

I had to resort to two separate fetches in order to achieve (I think) what you want. The first fetch gets objectIDs for one object for each distinct combination of country and city. The second fetch is filtered, using an IN predicate, to just these objects. It uses NSExpression and propertiesToGroupBy to get the counts for each country:
// Step 1, get the object IDs for one object for each distinct country and city
var objIDExp = NSExpression(expressionType: NSExpressionType.EvaluatedObjectExpressionType)
var objIDED = NSExpressionDescription()
objIDED.expression = objIDExp
objIDED.expressionResultType = .ObjectIDAttributeType
objIDED.name = "objID"
var fetch = NSFetchRequest(entityName: "Location")
fetch.propertiesToFetch = [objIDED]
fetch.propertiesToGroupBy = ["country", "city"]
fetch.resultType = .DictionaryResultType
let results = self.managedObjectContext!.executeFetchRequest(fetch, error: nil)
// extract the objectIDs into an array...
let objIDArray = (results! as NSArray).valueForKey("objID") as! [NSManagedObjectID];
// Step 2, count using GROUP BY
var countExp = NSExpression(format: "count:(SELF)")
var countED = NSExpressionDescription()
countED.expression = countExp
countED.expressionResultType = .ObjectIDAttributeType
countED.name = "count"
var newFetch = NSFetchRequest(entityName: "Location")
newFetch.predicate = NSPredicate(format: "SELF IN %#", objIDArray)
newFetch.propertiesToFetch = ["country", countED]
newFetch.propertiesToGroupBy = ["country"]
newFetch.resultType = .DictionaryResultType
let newResults = self.managedObjectContext!.executeFetchRequest(newFetch, error: nil)
println("\(newResults!)")
This will be inefficient: if you have a large number of distinct countries and cities, the IN predicate will slow things down. You might find it's more efficient to fetch everything and count them.