core data操作

 

    AppDelegate *appDelegate =

[[UIApplicationsharedApplication] delegate];

    NSManagedObjectContext *context = [appDelegate managedObjectContext];

   

   

    //classes

    NSEntityDescription *entityDescriptionForClass = [NSEntityDescription

  entityForName:@"ClassEntity"

  inManagedObjectContext:context];

    NSFetchRequest *requestForClass = [[NSFetchRequestalloc] init];

    [requestForClass setEntity:entityDescriptionForClass];

    NSError *error;

    NSArray *objectsForClass = [context executeFetchRequest:requestForClass error:&error];

    if (objectsForClass == nil) {

        NSLog(@"There was an error in my Zander App!");

        // Do whatever error handling is appropriate 可以在这里填写 当发生存取错误的时候 处理的内容

    }

//初始化一个entity对象 

 StudentEntity *student1= [[StudentEntity alloc]initWithEntity:entityDescription insertIntoManagedObjectContext:context];

 student1.name = @"student1_1";

 student1.student_class = class1;

 

//将set 转化成 可变数组mutablearray

 NSSet *set = selectedClass.class_student;

    NSSortDescriptor *sort = [NSSortDescriptorsortDescriptorWithKey:@"index"ascending:YES];

    

    NSArray *array = [set sortedArrayUsingDescriptors:[NSArray  arrayWithObject:sort]];    

    

    NSMutableArray *studentsArray = [NSMutableArrayarrayWithArray:array] ;

 

  

 

 

//降序取数据的 NSFetchRequest 声明

    NSFetchRequest *fetchRequest2 =[[NSFetchRequestalloc]init];

    NSEntityDescription *entity = [NSEntityDescriptionentityForName:@"EntityName" inManagedObjectContext:context];

    [fetchRequest2 setEntity:entity];

    [fetchRequest2 setSortDescriptors:[NSArrayarrayWithObject:[NSSortDescriptorsortDescriptorWithKey:@"displayOrder"ascending:NO]]];

 

 

主要用的就是这个 [fetchRequest2 setSortDescriptors:[NSArrayarrayWithObject:[NSSortDescriptorsortDescriptorWithKey:@"displayOrder"ascending:NO]]];

设置这个数据库取数据的请求 1. 按照那个字段  （displayOrder）   2. 排序：升序还是降序  YES （升序） NO（降序）