ios - NSPredicate 和 Core Data 多表查询

Question

我有一个 SQL 命令，使用普通 SQL 构建起来非常简单，但在使用 Core Data 使其工作时遇到了很多麻烦。

我有 3 个实体；项目、文件夹和组。项目位于文件夹中，文件夹位于组中。通过文件夹名称和组名称，文件夹是唯一的。项目只存储文件夹名称，而文件夹则存储文件夹名称和组名称。

有问题的 SQL 命令大致是这样的:SELECT ItemID FROM Item WHERE Item.folderName =folderName ANDFolder.folderName = Item.folderName ANDFolder.groupName = groupName;

(是的，我知道我没有通过 CoreData 获得特定属性，我只是为了示例而这样做)

当我执行 Folder.folderName = Item.folderName AND Folder.groupName = groupName 部分时，就会出现问题。我不知道如何将其表示为谓词。

我应该注意，项目和文件夹之间以及文件夹和组之间都有关系。

编辑:

这是我的数据模型的设置:

这是我现在用来发出请求的代码:

NSFetchRequest *request = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"FavouriteItem" inManagedObjectContext:context];

[request setEntity:entity];

request.predicate = [NSPredicate predicateWithFormat:@"folderName = %@ AND folderRelationship.groupName = %@", self.folderName, self.groupName];

NSError *fetchError;

NSArray *results = [context executeFetchRequest:request error:&fetchError];

Answer 1

因此，基本上，您想要获取特定文件夹中的所有项目(即，您想要具有给定文件夹名称和组名称的所有项目......它们唯一标识特定文件夹)。

既然你说你已经有了关系，我会假设它们是建立在反向关系上的。

此外，我将假设一个项目具有名为“文件夹”的一对一关系，即该项目所在的文件夹。该文件夹将与该文件夹中的项目具有一对多关系“项目”。

因此，以下将是实现您想要的目标的众多方法之一。

- (NSSet*)getItemsForFolderName:(NSString*)folderName
                      groupName:(NSString*)groupName
                          inMOC:(NSManagedObjectContext*)moc
                          error:(NSError**)error {
    NSSet *result = nil;
    NSFetchRequest *fetchRequest = [NSFetchRequest fetchRequestWithEntityName:@"Folder"];
    fetchRequest.predicate = [NSPredicate predicateWithFormat:@"folderName = %@ AND groupName = %@", folderName, groupName];
    fetchRequest.relationshipKeyPathsForPrefetching = @[@"items"];
    NSArray *folders = [moc executeFetchRequest:fetchRequest error:error];
    if (folders) {
        result = [[folders firstObject] items];
    }
    return result;
}

您也可以简单地获取项目。至于哪个最好，这取决于您如何为您的属性建立索引，以及您正在进行什么类型的其他抓取。

- (NSArray*)getItemsForFolderName:(NSString*)folderName
                        groupName:(NSString*)groupName
                            inMOC:(NSManagedObjectContext*)moc
                            error:(NSError**)error {
    NSFetchRequest *fetchRequest = [NSFetchRequest fetchRequestWithEntityName:@"Item"];
    fetchRequest.predicate = [NSPredicate predicateWithFormat:@"folderName = %@ AND folder.groupName = %@", folderName, groupName];
    return [moc executeFetchRequest:fetchRequest error:error];
}

编辑

JamEngulfer221，我很确定这些例子会起作用，但既然你说即使在我指出你帖子中的错误后它们也不起作用，我想我会搞一个快速的项目并测试......我'我老了，有时忘记东西...

因此，我使用以下代码破解了一个测试。

首先，创建模型...

- (NSManagedObjectModel *)makeModel {
    NSEntityDescription *group = [[NSEntityDescription alloc] init];
    group.name = @"Group";
    NSAttributeDescription *groupName = [[NSAttributeDescription alloc] init];
    groupName.name = @"groupName";
    groupName.attributeType = NSStringAttributeType;

    NSEntityDescription *folder = [[NSEntityDescription alloc] init];
    folder.name = @"Folder";
    NSAttributeDescription *folderName = [[NSAttributeDescription alloc] init];
    folderName.name = @"folderName";
    folderName.attributeType = NSStringAttributeType;

    NSEntityDescription *item = [[NSEntityDescription alloc] init];
    item.name = @"Item";
    NSAttributeDescription *itemName = [[NSAttributeDescription alloc] init];
    itemName.name = @"itemName";
    itemName.attributeType = NSStringAttributeType;

    NSRelationshipDescription *folderToGroupRelationship = [[NSRelationshipDescription alloc] init];
    NSRelationshipDescription *groupToFoldersRelationship = [[NSRelationshipDescription alloc] init];

    groupToFoldersRelationship.name = @"folders";
    groupToFoldersRelationship.destinationEntity = folder;
    groupToFoldersRelationship.minCount = 0;
    groupToFoldersRelationship.maxCount = 0;
    groupToFoldersRelationship.deleteRule = NSCascadeDeleteRule;
    groupToFoldersRelationship.inverseRelationship = folderToGroupRelationship;

    folderToGroupRelationship.name = @"group";
    folderToGroupRelationship.destinationEntity = group;
    folderToGroupRelationship.minCount = 0;
    folderToGroupRelationship.maxCount = 1;
    folderToGroupRelationship.deleteRule = NSNullifyDeleteRule;
    folderToGroupRelationship.inverseRelationship = groupToFoldersRelationship;

    NSRelationshipDescription *folderToItemsRelationship = [[NSRelationshipDescription alloc] init];
    NSRelationshipDescription *itemToFolderRelationship = [[NSRelationshipDescription alloc] init];

    folderToItemsRelationship.name = @"items";
    folderToItemsRelationship.destinationEntity = item;
    folderToItemsRelationship.minCount = 0;
    folderToItemsRelationship.maxCount = 0;
    folderToItemsRelationship.deleteRule = NSCascadeDeleteRule;
    folderToItemsRelationship.inverseRelationship = itemToFolderRelationship;

    itemToFolderRelationship.name = @"folder";
    itemToFolderRelationship.destinationEntity = folder;
    itemToFolderRelationship.minCount = 0;
    itemToFolderRelationship.maxCount = 1;
    itemToFolderRelationship.deleteRule = NSNullifyDeleteRule;
    itemToFolderRelationship.inverseRelationship = folderToItemsRelationship;

    group.properties = @[groupName, groupToFoldersRelationship];
    folder.properties = @[folderName, groupName, folderToGroupRelationship, folderToItemsRelationship];
    item.properties = @[itemName, folderName, itemToFolderRelationship];

    NSManagedObjectModel *model = [[NSManagedObjectModel alloc] init];
    model.entities = @[group, folder, item];
    return model;
}

以及搜索方法...

- (NSArray*)getItemsForFolderName:(NSString*)folderName
                        groupName:(NSString*)groupName
                            inMOC:(NSManagedObjectContext*)moc
                            error:(NSError**)error {
    NSFetchRequest *fetchRequest = [NSFetchRequest fetchRequestWithEntityName:@"Item"];
    fetchRequest.predicate = [NSPredicate predicateWithFormat:@"folderName = %@ AND folder.groupName = %@", folderName, groupName];
    return [moc executeFetchRequest:fetchRequest error:error];
}

还有测试...

- (void)testBlarg {
    NSManagedObjectModel *model = [self makeModel];
    NSPersistentStoreCoordinator *psc = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:model];
    [psc addPersistentStoreWithType:NSInMemoryStoreType configuration:nil URL:nil options:nil error:NULL];
    NSManagedObjectContext *moc = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSMainQueueConcurrencyType];
    moc.persistentStoreCoordinator = psc;

    for (int g = 0; g < 10; ++g) {
        NSManagedObject *group = [NSEntityDescription insertNewObjectForEntityForName:@"Group" inManagedObjectContext:moc];
        NSString *groupName = [NSString stringWithFormat:@"Group %02d", g];
        [group setValue:groupName forKey:@"groupName"];
        for (int f = 0; f < 10; ++f) {
            NSManagedObject *folder = [NSEntityDescription insertNewObjectForEntityForName:@"Folder" inManagedObjectContext:moc];
            NSString *folderName = [NSString stringWithFormat:@"Folder %02d", f];
            [folder setValue:folderName forKey:@"folderName"];
            [folder setValue:groupName forKey:@"groupName"];
            [folder setValue:group forKey:@"group"];
            for (int i = 0; i < 10; ++i) {
                NSManagedObject *item = [NSEntityDescription insertNewObjectForEntityForName:@"Item" inManagedObjectContext:moc];
                NSString *itemName = [NSString stringWithFormat:@"Item %02d (%@;%@)", i, folderName, groupName];
                [item setValue:itemName forKey:@"itemName"];
                [item setValue:folderName forKey:@"folderName"];
                [item setValue:folder forKey:@"folder"];
            }
        }
    }
    [moc save:NULL];
    [moc reset];

    NSString *folderName = @"Folder 04";
    NSString *groupName = @"Group 02";
    NSArray *items = [self getItemsForFolderName:folderName groupName:groupName inMOC:moc error:NULL];
    items = [items sortedArrayUsingDescriptors:@[[NSSortDescriptor sortDescriptorWithKey:@"itemName" ascending:YES]]];
    XCTAssertEqual(10, items.count);
    for (int i = 0; i < 10; ++i) {
        NSManagedObject *object = items[i];
        NSString *expectedItemName = [NSString stringWithFormat:@"Item %02d (%@;%@)", i, folderName, groupName];
        XCTAssertEqualObjects(expectedItemName, [object valueForKey:@"itemName"]);
        NSManagedObject *folder = [object valueForKey:@"folder"];
        XCTAssertEqualObjects(folderName, [folder valueForKey:@"folderName"]);
        XCTAssertEqualObjects(groupName, [folder valueForKey:@"groupName"]);
        NSManagedObject *group = [folder valueForKey:@"group"];
        XCTAssertEqualObjects(groupName, [group valueForKey:@"groupName"]);
    }
}