报错：
Terminating app due to uncaught exception 'NSUnknownKeyException', reason: '[<__NSDictionary0 0x600000000de0> setValue:forUndefinedKey:]: this class is not key value coding-compliant for the key doctorCode.'
解决：
NSDictionary *params = [[NSDictionary alloc] init];
[params setValue:doctorCode forKey:@"doctorCode"];
改为
NSMutableDictionary *params = [[NSMutableDictionary alloc] init];
[params setValue:doctorCode forKey:@"doctorCode"];
bug消失，访问数据成功！！！
应该是NSDictionary不支持为未知的key赋值，只能是它原有的key才可以。
这就是基础知识的不足导致的失误！！！
