为获取== entity.attribute @＆QUOT请求; someValue中＆QUOT; [英] fetch request for entity.attribute == @"somevalue"

问题描述

我如何设置一个读取请求只与一个特定的值拉离实体的属性的数据？这是我以前所使用的基本code。

How do I setup a fetch request to only pull the data from an entity's attribute with one particular value? This is the basic code I've used before.

-(void)fetchResults
{
    NSFetchRequest *fetchRequest = [NSFetchRequest fetchRequestWithEntityName:self.entityDescription.name];
    NSString *cacheName = [self.entityDescription.name stringByAppendingString:@"Cache"];

    // predicate code
    if (self.predicate != nil) {
        [fetchRequest setPredicate:self.predicate];
    }
    // end of predicate code

    NSSortDescriptor *sortDescriptor = [NSSortDescriptor sortDescriptorWithKey:@"name" ascending:YES];
    [fetchRequest setSortDescriptors:[NSArray arrayWithObject:sortDescriptor]];

    self.fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:self.managedObjectContext sectionNameKeyPath:nil cacheName:cacheName];

    BOOL success;
    NSError *error;
    success = [self.fetchedResultsController performFetch:&error];
    if (!success)
    {
        NSLog(@"%@", [error localizedDescription]);
    }
}

我一直在看这个页面： http://bit.ly/KevYwR 这是正确的方向？

我是否需要使用 NS predicate 或者我可以不用？

Do I need to use NSPredicate or can I do without?

感谢您的帮助，在正确的方向点，等等。

Thanks for any help, point in the right direction, etc.

推荐答案

设置一个 NSFetchRequest 相当于SQL语言的SELECT statetement。

Setting up a NSFetchRequest is equivalent to a SELECT statetement in SQL language.

下面一个简单的例子：

NSFetchRequest *request = [[NSFetchRequest alloc] init];
[request setEntity:[NSEntityDescription entityForName:@"EntityName" inManagedObjectContext:moc]];

NSError *error = nil;
NSArray *results = [moc executeFetchRequest:request error:&error];

// error handling code

阵列结果包含所有包含源码文件中的管理对象。如果你想抓住特定对象（或多个对象），你需要使用predicate这一请求。例如：

The array results contains all the managed objects contained within the sqlite file. If you want to grab a specific object (or more objects) you need to use a predicate with that request. For example:

NSPredicate *predicate = [NSPredicate predicateWithFormat:@"attribute == %@", @"Some Value"];
[request setPredicate:predicate]; 

在这种情况下，结果包含其中，属性等于某个值的对象。设置predicate等于把在WHERE子句中的SQL语句。

In this case results contains the objects where attribute is equal to Some Value. Setting a predicate is equal to put the WHERE clause in a SQL statement.

注意

我假设实体的名称是实体名称和它的属性被称为属性这是字符串类型

I suppose that the name of the entity is EntityName and its property is called attribute which is of type string.

有关进一步信息，我建议你阅读核心数据编程指南和 NSFecthRequest 类引用。

For further info I suggest you to read the Core Data programming guide and NSFecthRequest class reference.

• <一个href=\"http://developer.apple.com/library/iOS/#documentation/Cocoa/Conceptual/CoreData/cdProgrammingGuide.html\" rel=\"nofollow\">http://developer.apple.com/library/iOS/#documentation/Cocoa/Conceptual/CoreData/cdProgrammingGuide.html

<一个href=\"https://developer.apple.com/library/mac/#documentation/Cocoa/Reference/CoreDataFramework/Classes/NSFetchRequest_Class/NSFetchRequest.html\" rel=\"nofollow\">https://developer.apple.com/library/mac/#documentation/Cocoa/Reference/CoreDataFramework/Classes/NSFetchRequest_Class/NSFetchRequest.html

希望它帮助。

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