Golang MongoDB (mgo) Find reflection error

Question

With the following code

func (s Store) Lookup(department string, number string) (*types.Course, error) {
    var result *types.Course
    err := s.collection.Find(bson.M{
        "department":    department,
        "course_number": number,
    }).One(result)

    if err != nil {
        switch err {
        case mgo.ErrNotFound:
            return nil, ErrNotFound
        default:
            log.Error(err)
            return nil, ErrInternal
        }
    }

    return result, nil
}

I came across the error:

reflect: reflect.Value.Set using unaddressable value

If I change the first line from var result *types.Course to result := &types.Course{} there is no error. What exactly is the difference between these two?

Answer 1

The two otions both declare a variable of type *types.Course. The first pointer value is nil. The second is initialized to point at a value of type types.Course.

 var result *types.Course    // result == nil
 result := &types.Course{}   // result != nil, points to a value.
 result := new(types.Course) // basically the same as the second

The mgo function requires a pointer to a value. A nil pointer does not point to a value.

The typical way to write this code is:

var result types.Course   // declare variable of result type, not a pointer
err := s.collection.Find(bson.M{
    "department":    department,
    "course_number": number,
}).One(&result)           // pass address of value to function