Saving enumerated values to a database

Question

I'm new to Go and I'm trying to write a little program to save enumerated values to a database. The way I declare my values is as follows:

type FileType int64
const (
    movie FileType = iota
    music
    book
    etc
)

I use these values in my struct like this:

type File struct {
    Name     string
    Type     FileType
    Size     int64
}

I use gorp for my database stuff, but I guess the use of gorp isn't relevant to my problem. I put stuff in my DB like this:

dbmap.Insert(&File{"MyBook.pdf",movie,1000})

but when I try to retrieve stuff…

dbmap.Select(&dbFiles, "select * from Files")

I get the following error:

panic: reflect.Set: value of type int64 is not assignable to type main.FileType

When I use int64 as the type for the const(...) and for the File.Type field, everything works fine, but I'm new to Go and want to understand the problem. The way I see it, I have two problems:

1. Why can't Go convert this stuff successfully? I looked at the source code of the Go reflection and sql packages and there are methods for this kind of conversion, but they seem to fail. Is this a bug? What is the problem?

2. I figured out, that one can implement the sql.Scanner interface by implementing the following method:

   Scan(src interface{}) error
   

   I tried to implement the method and I even was able to get the right value from src and convert it to a FileType, but I was confused if I should implement the method for "(f *FileType) or (f FileType). Either way the method gets invoked, however I'm not able to overwrite f (or at least the update gets lost later) and the File instances read from the DB always had a "0" as value for File.Type.

Do you have any ideas on those two points?

Answer 1

I recently had the same need, and the solution is to implement two interfaces:

1. sql/driver.Valuer
2. sql.Scanner

Here's a working example:

type FileType int64

func (u *FileType) Scan(value interface{}) error { *u = FileType(value.(int64)); return nil }
func (u FileType) Value() (driver.Value, error)  { return int64(u), nil }

Answer 2

Slightly off-topic, but may be useful to others as I kept revisiting this question/answer when solving a similar problem when working with postgres enum fields in golang (which are returned as bytes).

 // Status values
 const ( 
     incomplete Status = "incomplete"
     complete   Status = "complete" 
     reject     Status = "reject"
 )

 type Status string

 func (s *Status) Scan(value interface{}) error {
     asBytes, ok := value.([]byte)
     if !ok {
         return errors.New("Scan source is not []byte")
     }
     *s = Status(string(asBytes))
     return nil
 }

 func (s SubjectStatus) Value() (driver.Value, error) {
     // validation would go here
     return string(s), nil
 }