Question

我有一个SQL命令，使用普通的SQL构建是非常简单的，但是我在使用Core Data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使文件夹唯一。项目只存储文件夹名称，而文件夹存储文件夹名称和组名称。

有问题的SQL命令大致如下：SELECT ItemID FROM Item WHERE Item.folderName = folderName AND Folder.folderName = Item.folderName AND Folder.groupName = groupName;

（是的，我知道我没有得到CoreData的特定属性，我只是为了这个例子而做）

当我执行Folder.folderName = Item.folderName AND Folder.groupName = groupName部分时出现问题。我无法弄清楚如何将其表示为谓词。

我应该注意，我在项目和文件夹之间存在关系，以及文件夹和组之间的关系。

编辑：

这是我的数据模型的设置：

这是我现在用于提出请求的代码：

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，您希望所有具有给定folderName和groupName的项目...唯一标识特定文件夹。）

既然你说你已经有了关系，我会假设它们是以反向关系设置的。

此外，我将假设一个项目具有一个名为＆＃34;文件夹＆＃34;的一对一关系。该项目所在的文件夹。该文件夹将具有多对多关系＆＃34; items＆＃34;对于该文件夹中的项目。

因此，以下是实现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这些项目。至于哪个最好，这取决于你如何索引你的属性，以及你正在做什么类型的其他提取。

- (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];
}

修改

JamEngulfer221，我很确定这些例子是可行的，但是既然你说他们甚至在我指出你的帖子中的错误之后也没有，我想我会快速完成一个项目并进行测试。 ..我老了，有时会忘记......

所以，我用以下代码破解了测试。

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; }

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]; }

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"]); } }

Answer 2

我认为您的数据模型设置存在缺陷，导致这种混淆。实际上，你的问题很简单。

首先，如果项目被分配到文件夹，则不需要存储文件夹名称 - 这将是多余的，因此是不必要的信息。这同样适用于文件夹和组名称。

其次，父文件夹应该是与同一实体的自反关系，而不是属性。

所有实体应该只有name属性和这些关系（请注意直观的非技术关系名称）：

Group (folders)     <------>> (group) Folder (items) <------->> (folder) Item
Folder (subFolders) <------>> (parentFolder) Folder

谓词现在很简单：

[NSPredicate predicateWithFormat:
  @"folder.name = %@ && folder.group.name = %@", folderName, groupName]

NSPredicate和Core Data多表查询

2 个答案: