coredata 查询操作的核心代码

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];
}