对许多人的核心数据谓词答案

【问题标题】：Core Data Predicate To Many对许多人的核心数据谓词
【发布时间】：2012-10-31 22:35:50
【问题描述】：

我有一个核心数据模型，它具有一对多关系，有一个类别，它可以包含许多子类别。

类别 > 子类别

我正在尝试执行获取，以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，我想获取名称为“水果”的类别中是否有名称为“苹果”的子类别。

 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;
    }
}

【问题讨论】：

:) 这对我来说看起来不错，所以你的问题是什么？
NSPredicate *predicate = [NSPredicate predicateWithFormat:@"name == [c] %@ and category == %@", subCategory, category];

标签： ios core-data one-to-many

【解决方案1】：

尝试寻找具有您喜欢的子类别的类别实体，而不是相反，然后从结果中返回 category.subcategory。我不确定你的 NSManagedObjects 是什么样子，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;
    }
}

...或者只是将类别添加到您的谓词中，您应该将它包含在您的子类别托管对象定义中：

- (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;
        }
    }

【讨论】：