核心数据NSFetchRequest还获取实体的子对象 [英] Core data NSFetchRequest also fetches children objects of the Entity

问题描述

我是iOS dev和Core Data的新手。我有一个父NSManagedObject

I'm new to iOS dev and Core Data. I have a parent NSManagedObject

@class Units;
@interface Properties : NSManagedObject

@property (nonatomic, retain) NSString * descr;
@property (nonatomic, retain) NSString * address;
@property (nonatomic, retain) NSString * city;
@property (nonatomic, retain) NSString * state;
@property (nonatomic, retain) NSString *zipCode;
@property (nonatomic, retain) NSString * imageKey;
@property (nonatomic, retain) NSString * numberOfUnit;
@property (nonatomic, retain) NSData * thumbnailData;
@property (nonatomic, strong) UIImage * thumbnail;
@property (nonatomic) double orderingValue;
@property (nonatomic, retain) NSSet *units;

和一个孩子：

@class Properties;

@interface Units : Properties

@property (nonatomic, retain) NSString * unitDescr;
@property (nonatomic) int16_t unitNumber;
@property (nonatomic, retain) Properties *property;

当我使用此方法获取父属性以在tableview中显示父属性对象时：

When I fetch the Parent Properties with this method to display the parent Properties objects in a tableview:

- (void)loadAllItems
{
    if (!allItems) {
        NSFetchRequest *request = [[NSFetchRequest alloc] init];

        NSEntityDescription *e = [[model entitiesByName] objectForKey:@"Properties"];
        [request setEntity:e];

        NSSortDescriptor *sd = [NSSortDescriptor
                                sortDescriptorWithKey:@"orderingValue"
                                ascending:YES];
        [request setSortDescriptors:[NSArray arrayWithObject:sd]];

        NSError *error;
        NSArray *result = [context executeFetchRequest:request error:&error];
        if (!result) {
            [NSException raise:@"Fetch failed"
                        format:@"Reason: %@", [error localizedDescription]];
        }

        allItems = [[NSMutableArray alloc] initWithArray:result];
    }
}

我遇到核心数据上下文的问题获取父实体的子对象。我只想返回父对象。

I run in to a problem where the core data context fetches the child objects of the Parent Entity. I just want to return the Parent Objects.

例如，如果我有一个包含3个单位的属性，则属性tableview应该只显示1行，但它显示4行（1个父节点和3个子节点）。

For example, If I have a property with 3 units, the properties tableview should display just 1 row but it is displaying 4 rows (1 parent and 3 childs).

如何返回父对象？

谢谢。

推荐答案

如果您将属性定义为 Units 的父实体，那么每个 Units object也是 Properties 对象。因此，获取属性也会返回所有单位。

If you define Properties as parent entity of Units, then every Units object is also a Properties object. Therefore fetching properties returns also all units.

这可能不是您想要的。您应该只定义 Properties 和 Units 之间的关系，而无需设置父实体（这样两个类都是 NSManagedObject的直接子类）。

This is probably not what you wanted. You should just define the relationships between Properties and Units, without setting a parent entity (so that both classes are direct subclasses of NSManagedObject).

备注：我会调用实体 Property 和 Unit ，因为每个实例代表一个属性或单元。

Remark: I would call the entities Property and Unit, because each instance represents a single property or unit.

这篇关于核心数据NSFetchRequest还获取实体的子对象的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！