Fetch duplicate records with CoreData

CoreDataDuplicateRecordsPlayground.swift

// Playground is where kids play

import CoreData

@objc(City)
class City: NSManagedObject {
    @NSManaged var name: String
}

var cityEntity = NSEntityDescription()
cityEntity.name = "City"
cityEntity.managedObjectClassName = "City"

var nameAttribute = NSAttributeDescription()
nameAttribute.name = "name"
nameAttribute.attributeType = NSAttributeType.StringAttributeType
nameAttribute.optional = false
nameAttribute.indexed = true

cityEntity.properties = [nameAttribute]

var model = NSManagedObjectModel()
model.entities = [cityEntity]

var persistentStoreCoordinator = NSPersistentStoreCoordinator(managedObjectModel: model)
let url = NSURL(fileURLWithPath: "database.sqlite")

// destroy store on each run
try persistentStoreCoordinator.destroyPersistentStoreAtURL(url, withType: NSSQLiteStoreType, options: nil)

do {
    try persistentStoreCoordinator.addPersistentStoreWithType(NSSQLiteStoreType, configuration: nil, URL: url, options: nil)
}
catch {
    print("error creating psc: \(error)")
}

var managedObjectContext = NSManagedObjectContext(concurrencyType: NSManagedObjectContextConcurrencyType.MainQueueConcurrencyType)
managedObjectContext.persistentStoreCoordinator = persistentStoreCoordinator

for i in 0 ..< 5 {
    for j in 0 ..< 5 {
        let city = City(entity: cityEntity, insertIntoManagedObjectContext: managedObjectContext)

        if i == 4 && j > 0 {
            continue
        }
        
        city.name = "City \(i)"

        try? managedObjectContext.save()
    }
}

let fetchRequest = NSFetchRequest(entityName: "City")

let nameExpr = NSExpression(forKeyPath: "name")
let countExpr = NSExpressionDescription()
let countVariableExpr = NSExpression(forVariable: "count")

countExpr.name = "count"
countExpr.expression = NSExpression(forFunction: "count:", arguments: [ nameExpr ])
countExpr.expressionResultType = .Integer64AttributeType

fetchRequest.resultType = .DictionaryResultType
fetchRequest.sortDescriptors = [ NSSortDescriptor(key: "name", ascending: true) ]
fetchRequest.propertiesToGroupBy = [ cityEntity.propertiesByName["name"]! ]
fetchRequest.propertiesToFetch = [ cityEntity.propertiesByName["name"]!, countExpr ]
fetchRequest.havingPredicate = NSPredicate(format: "%@ > 1", countVariableExpr)

let results = try managedObjectContext.executeFetchRequest(fetchRequest)

print("\(results)")