coredata 查询操作的核心代码_查询核心代码-优快云博客

本文介绍了一个使用 Core Data 进行复杂查询的例子。通过 NSFetchRequest 和 NSPredicate 设置查询条件，根据 dishClass 字段筛选出“川菜”类别，并按 dishCount 字段进行降序排序。

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NSFetchRequest *request = [[NSFetchRequest alloc] init];
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"Order" inManagedObjectContext:managedObjectContext];
    [request setEntity:entity];

    //查询条件
    NSString *str = @:"川菜";

    NSPredicate *predicate = [NSPredicate predicateWithFormat:@"dishClass = %@",str];

    [request setPredicate:predicate];//设置查询条件，即查询所有dishClass为川菜的数据

    //依照dishCount对查询结果排序
    NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"dishCount" ascending:NO];
    NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
    
    [request setSortDescriptors:sortDescriptors];
    
    NSError *error = nil;
    //获得一个包含order对象的数组
    NSMutableArray *mutableFetchResults = [[managedObjectContext executeFetchRequest:request error:&error] mutableCopy];

    //遍历取出数据
    for (int j = 0;j < mutableFetchResults.count; j++) {
        
        Order *order = [mutableFetchResults objectAtIndex:j];
        
        NSString *name = order.dishName;
        NSString *class = order.dishClass;
        NSString *count = [NSString stringWithFormat:@"%d",order.dishName];
    }