Orm-Read-QuerySetter-No-Rows

New feature has been using ORM instead of Redis, json marshal && unmarshal. But there's little problem when I wrote a model like this

type A struct {
  Id int64 `orm:"auto"`
  UId int64
  FId int64
  Time time.Time
}

func GetA(uid, fid int64) *A {
  o := orm.NewOrm()
  a := A{
    UId: uid,
    FId: fid,
  }
  if err := o.Read(&a); err == nil {
    return &a
  }
  return nil
}

GetA return nil all the time, I've been tested using model User like this

type User struct {
  Id int64 `orm:"auto"`
  Time time.Time
}

func GetUser(uid, fid int64) *User {
  o := orm.NewOrm()
  u := User{
    Id: uid,
  }
  if err := o.Read(&u); err == nil {
    return &u
  }
  return nil
}

And GetUser works well. Before I wonder whether need to use another way to make it work, I googled this problem, and found the answer is that:

When you use orm's Read, you need provide the primary key field or it will not return the result you want.

So GetUser works as Id is primary key and argument is the value passed to it, but GetA don't as Id the primary key field not set.