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コアデータ-最大値プロパティを持つエンティティをフェッチする方法

プロパティPersonのエンティティpersonIdがあります（personIdは一意です）

最大のpersonIdを持つPersonを取得するにはどうすればよいですか？

（私はプロパティの値ではなく人自体をフェッチしたいです）

iphoneobjective-cioscore-datansfetchedresultscontroller
39
Eyal

fetchLimitを1に設定し、personIdで降順に並べ替えます。例えば。：

NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] initWithEntityName:@"Person"];

fetchRequest.fetchLimit = 1;
fetchRequest.sortDescriptors = @[[NSSortDescriptor sortDescriptorWithKey:@"personId" ascending:NO]];

NSError *error = nil;

id person = [managedObjectContext executeFetchRequest:fetchRequest error:&error].firstObject;
68
hypercrypt

クエリを指定するには、NSFredicateでNSFe​​tchRequestを使用する必要があります...

AppleのPredicate Progamming Guideからの抜粋：

NSFetchRequest *request = [[[NSFetchRequest alloc] init] autorelease];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Person"
    inManagedObjectContext:managedObjectContext];
[request setEntity:entity];

request.predicate = [NSPredicate predicateWithFormat:@"personId==max(personId)"];
request.sortDescriptors = [NSArray array];

NSError *error = nil;
NSArray *array = [managedObjectContext executeFetchRequest:request error:&error];
23
Uilleann

推奨される方法は Apple Recommended Method NSExpressionを使用することです。これはソートを使用するよりも安価になると思います。ソートについて考えると、ソートを使用する場合、すべてのレコードをソートして最大のものを維持する必要があります。式を使用する場合は、リストを読んで最大値をメモリに保持する必要があります。

これは私がNSDateで使用する例です

- (NSDate *)lastSync:(PHAssetMediaType)mediaType {
    NSEntityDescription *entity = [NSEntityDescription  entityForName:kMediaItemEntity inManagedObjectContext:self.managedObjectContext];

    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    fetchRequest.entity = entity;
    fetchRequest.resultType = NSDictionaryResultType;

    NSMutableArray *predicates = [NSMutableArray array];
    [predicates addObject:[NSPredicate predicateWithFormat:@"%K=%d", kMediaType,mediaType]];
    [predicates addObject:[NSPredicate predicateWithFormat:@"%K=%d", kMediaProviderType,self.mediaProviderType]];
    NSPredicate *predicate = [NSCompoundPredicate andPredicateWithSubpredicates: predicates];
    fetchRequest.predicate = predicate;

    // Create an expression for the key path.

    NSExpression *keyPathExpression = [NSExpression expressionForKeyPath:kSyncTime];
    // Create an expression to represent the function you want to apply

    NSExpression *maxExpression = [NSExpression expressionForFunction:@"max:"
                                                            arguments:@[keyPathExpression]];

    // Create an expression description using the maxExpression and returning a date.
    NSExpressionDescription *expressionDescription = [[NSExpressionDescription alloc] init];
    [expressionDescription setName:@"maxDate"];
    [expressionDescription setExpression:maxExpression];
    [expressionDescription setExpressionResultType:NSDateAttributeType];

    // Set the request's properties to fetch just the property represented by the expressions.
    fetchRequest.propertiesToFetch = @[expressionDescription] ; // @[kSyncTime];

    NSError *fetchError = nil;
    id requestedValue = nil;

    // fetch stored media
    NSArray *results = [self.managedObjectContext executeFetchRequest:fetchRequest error:&fetchError];
    if (fetchError || results == nil || results.count == 0) {
        return [NSDate dateWithTimeIntervalSince1970:0];
    }
    requestedValue = [[results objectAtIndex:0] valueForKey:@"maxDate"];
    if (![requestedValue isKindOfClass:[NSDate class]]) {
        return [NSDate dateWithTimeIntervalSince1970:0];
    }
    DDLogDebug(@"sync date %@",requestedValue);
    return (NSDate *)requestedValue;
}
17
Ryan Heitner

NSExpressionを使用した上記の答えは正しいです。 Swiftバージョンです。

private func getLastSyncTimestamp() -> Int64? {

let request: NSFetchRequest<NSFetchRequestResult> = NSFetchRequest()
request.entity = NSEntityDescription.entity(forEntityName: "EntityName", in: self.moc)
request.resultType = NSFetchRequestResultType.dictionaryResultType

let keypathExpression = NSExpression(forKeyPath: "timestamp")
let maxExpression = NSExpression(forFunction: "max:", arguments: [keypathExpression])

let key = "maxTimestamp"

let expressionDescription = NSExpressionDescription()
expressionDescription.name = key
expressionDescription.expression = maxExpression
expressionDescription.expressionResultType = .integer64AttributeType

request.propertiesToFetch = [expressionDescription]

var maxTimestamp: Int64? = nil

do {

    if let result = try self.moc.fetch(request) as? [[String: Int64]], let dict = result.first {
       maxTimestamp = dict[key]
    }

} catch {
    assertionFailure("Failed to fetch max timestamp with error = \(error)")
    return nil
}

return maxTimestamp
}

ここで、mocはNSManagedObjectContextです。

7
jarora

スウィフト3

let request:NSFetchRequest = Person.fetchRequest()

let sortDescriptor1 = NSSortDescriptor(key: "personId", ascending: false)

request.sortDescriptors = [sortDescriptor1]

request.fetchLimit = 1

do {
    let persons = try context.fetch(request)
    return persons.first?.personId
} catch {
    print(error.localizedDescription)
}
2
Rajat Jain

スウィフト4

let request: NSFetchRequest<Person> = Person.fetchRequest()
request.fetchLimit = 1

let predicate = NSPredicate(format: "personId ==max(personId)")
request.predicate = predicate

var maxValue: Int64? = nil
do {
    let result = try self.context.fetch(request).first
    maxValue = result?.personId
} catch {
    print("Unresolved error in retrieving max personId value \(error)")
}
1
hamada147