Question

我有一个具有一对多关系的核心数据模型，有一个类别，它可以包含许多子类别。

类别＆lt; ----＆gt;＆gt;子类别

我正在尝试执行一个提取，检查特定类别是否包含具有特定名称的子类别。假设我下面有两个类别，我想要查看在名为“Fruits”的类别中是否有任何子类别名称“Apple”。

 Vetegables
    - Carrot
    - Lettuce

 Fruits
    - Apple
    - Orange
    - Pear

代码：

- (SubCategory *)searchForSubCategoryWithName:(NSString *)subCategory
                                   inCategory:(Category *)category
{
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"SubCategory" inManagedObjectContext:self.beer.managedObjectContext];
    [fetchRequest setEntity:entity];

    NSPredicate *predicate = [NSPredicate predicateWithFormat:@"name == [c] %@", subCategory];
    [fetchRequest setPredicate:predicate];

    NSError *error;
    NSArray *fetchedObjects = [self.beer.managedObjectContext executeFetchRequest:fetchRequest error:&error];

    if (fetchedObjects != nil && fetchedObjects.count > 0) {
        return [fetchedObjects objectAtIndex:0];
    }
    else {
        return nil;
    }
}

Answer 1

尝试查找具有您喜欢的子类别的类别实体，然后从结果中返回category.subcategory。我不确定你的NSManagedObjects是什么样的，但试试这个：

- (SubCategory *)searchForSubCategoryWithName:(NSString *)subCategory
                                   inCategory:(Category *)category
{
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"Category" inManagedObjectContext:self.beer.managedObjectContext];
    [fetchRequest setEntity:entity];

    NSPredicate *predicate = [NSPredicate predicateWithFormat:@"subcategory.name == [c] %@ AND name == %@", subCategory, category.name];
    [fetchRequest setPredicate:predicate];

    NSError *error;
    NSArray *fetchedObjects = [self.beer.managedObjectContext executeFetchRequest:fetchRequest error:&error];

    if (fetchedObjects != nil && fetchedObjects.count > 0) {
        Category *categoryManagedObject = [fetchedObjects objectAtIndex:0];
        return categoryManagedObject.subcategory;
    }
    else {
        return nil;
    }
}

...或者只是在谓词中添加类别，您应该在子类别托管对象定义中添加它：

- (SubCategory *)searchForSubCategoryWithName:(NSString *)subCategory
                                       inCategory:(Category *)category
    {
        NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
        NSEntityDescription *entity = [NSEntityDescription entityForName:@"Category" inManagedObjectContext:self.beer.managedObjectContext];
        [fetchRequest setEntity:entity];

        NSPredicate *predicate = [NSPredicate predicateWithFormat:@"name == [c] %@ AND category == %@", subCategory, category];
        [fetchRequest setPredicate:predicate];

        NSError *error;
        NSArray *fetchedObjects = [self.beer.managedObjectContext executeFetchRequest:fetchRequest error:&error];

        if (fetchedObjects != nil && fetchedObjects.count > 0) {
            return [fetchedObjects objectAtIndex:0];
        }
        else {
            return nil;
        }
    }

核心数据对许多人的预测

1 个答案: