查找或创建独特的核心数据实体
核心数据驱动应用程序的常见场景是从后备存储中获取唯一的对象。如果存在具有某一唯一属性的对象，则返回该对象，如果不返回新创建的对象。我发现自己一遍又一遍地写同样的东西，所以我用一种方便的方法包装它。但这看起来太微不足道了，我是不是在重新发明轮子？有没有一种更简单、开箱即用的方法来实现这一目标？
干杯，
EP
+(id)uniqueEntityfForName:(NSString *)name
withValue:(id)value
forKey:(NSString *)key
inManagedObjectContext:(NSManagedObjectContext *)context {
NSFetchRequest *request = [[[NSFetchRequest alloc] init] autorelease];
request.entity = [NSEntityDescription entityForName:name inManagedObjectContext:context];
request.predicate = [NSPredicate predicateWithFormat:[key stringByAppendingString:@" == %@"], value];
NSArray *result = [context executeFetchRequest:request error:nil];
id entity = [result lastObject];
if (entity == nil) {
entity = [NSEntityDescription insertNewObjectForEntityForName:name inManagedObjectContext:context];
[entity setValue:value forKey:key];
} else {
entity = [result lastObject];
}
return entity;
}
我使用这样的方法：
SomeEntity *entity = [CDUtils uniqueEntityfForName:@"SomeEntity" withValue:@"foo" forKey:@"bar" inManagedObjectContext:context];
A common scenario for core data driven apps is getting a unique object out of the backing store. If the object with a certain unique property exists, return that one, if it doesn't return a newly create one. I found myself writing the same thing over and over, so I wrapped it in a convenience method. But this seems so trivial, am I reinventing the wheel here? Is there a simpler, out-of-the-box way to achieve this?
Cheers,
EP
+(id)uniqueEntityfForName:(NSString *)name
withValue:(id)value
forKey:(NSString *)key
inManagedObjectContext:(NSManagedObjectContext *)context {
NSFetchRequest *request = [[[NSFetchRequest alloc] init] autorelease];
request.entity = [NSEntityDescription entityForName:name inManagedObjectContext:context];
request.predicate = [NSPredicate predicateWithFormat:[key stringByAppendingString:@" == %@"], value];
NSArray *result = [context executeFetchRequest:request error:nil];
id entity = [result lastObject];
if (entity == nil) {
entity = [NSEntityDescription insertNewObjectForEntityForName:name inManagedObjectContext:context];
[entity setValue:value forKey:key];
} else {
entity = [result lastObject];
}
return entity;
}
I use this method like this:
SomeEntity *entity = [CDUtils uniqueEntityfForName:@"SomeEntity" withValue:@"foo" forKey:@"bar" inManagedObjectContext:context];
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评论（2）
相当标准。我的核心数据实体有很多方法，例如
[aStudent enrollmentForId:(long long)idValue createIfMissing:YES]。我还想插入 mogenerator ，这可以消除核心数据的很多痛苦。除此之外，它还为数据模型中定义的每个获取请求生成一个工厂方法。因此，在模型中创建一个获取谓词，例如，
产生一个匹配的类方法：
...它执行您在那里编写的前半部分内容。 类似的包装器都很简单。
这样，在任何包含托管对象上下文的类中（例如“父”实体或应用程序委托等），编写
Pretty standard. My core data entities have lots of methods like
[aStudent enrollmentForId:(long long)idValue createIfMissing:YES].I'd also like to plug mogenerator , which removes much pain from Core Data. Among other things, it generates a factory method for every fetch request defined in the data model. So making a fetch predicate in the model like, e.g,
yields a matching class method:
...which does the first half of what you've written up there. A wrapper like
is then trivial to write, in whatever class holds your managedObjectContext (eg. a "parent" entity, or app delegate, or whatever.)
更灵活的解决方案是使用 Blocks 让调用者在比较两个列表时处理 3 种情况。
因此，在以同步方式插入或添加到数据存储时无需创建类似的函数。
完整的实现可以在这里找到： http:// /emplementation.blogspot.com/2011/12/importing-data-into-core-data-while.html
A more flexible solution is to use
Blocksto let the caller to handle 3 situations when comparing two lists.Therefore, there is no need to create similar functions when inserting in a syncing fashion or additions to the data store.
A full implementation can be found here: http://emplementation.blogspot.com/2011/12/importing-data-into-core-data-while.html