Getting the value of sql.NullString with reflection

Question

I'd like to get the value of all the fields in a struct that aren't empty. This works for simple primitives (string, int, etc.), but not for struct types such as sql.NullString.

Very simple example:

package main

import (
    "database/sql"
    "fmt"
    "reflect"
)

func main() {
    type fooT struct {
        NullS sql.NullString
    }

    values := reflect.ValueOf(fooT{})

    field := values.Field(0)
    v := reflect.ValueOf(field)

    iface := v.Interface().(sql.NullString)

    fmt.Println(iface)
}

This gives a panic:

panic: interface conversion: interface is reflect.Value, not sql.NullString

I don't understand this, since the Interface method should return an interface{} (and not a reflect.Value) which I can then type assert (?)

First I thought I was maybe using type conversions only works for primitives, but a quick test script:

package main

import (
    "database/sql"
    "fmt"
)

func main() {
    type fooT struct {
        NullS sql.NullString
    }

    foo := fooT{NullS: sql.NullString{"It's an Aardvark!", true}}

    var iface interface{}
    iface = foo.NullS

    fmt.Printf("%T -> %#v\n", iface, iface)
    fmt.Printf("%T -> %#v -> %#v\n", iface.(sql.NullString), iface.(sql.NullString),
        iface.(sql.NullString).Valid)
}

Reveals that this should work?

The full code I'm using:

package main

import (
    "database/sql"
    "fmt"
    "reflect"
)

type fooT struct {
    ID    int64
    Foo   string
    NullS sql.NullString
    FooQ  sql.NullString
}

func main() {
    foo := fooT{
        ID:    42,
        NullS: sql.NullString{"Your mother was a hamster", true},
    }

    types := reflect.TypeOf(foo)
    values := reflect.ValueOf(foo)

    changed := ""
    for i := 0; i < types.NumField(); i++ {
        fieldType := types.Field(i)
        field := values.Field(i)

        switch field.Type().Kind() {
        // Works
        case reflect.String:
            if field.String() != "" {
                changed += fmt.Sprintf("<strong>%s</strong>: %v<br>\n",
                    fieldType.Name, field.String())
            }
        default:
            switch field.Type().String() {
            case "sql.NullString":
                v := reflect.ValueOf(field)

                // NullS: reflect.Value -> sql.NullString{String:"Your mother was a hamster", Valid:true}
                iface := v.Interface()
                fmt.Printf("%s: %T -> %#v\n",
                    fieldType.Name, iface, iface)

                // panic: interface conversion: interface is reflect.Value, not sql.NullString
                iface2 := v.Interface().(sql.NullString)
                fmt.Printf("%s: %T -> %#v\n",
                    fieldType.Name, iface2, iface2)
            }
        }
    }

    fmt.Printf(changed)
}

Thanks for including a complete program. I think you don't need that reflect.ValueOf in the NullString case (which, note, you don't have for the string case). I may be misunderstanding the question but I'll post an answer with a couple code samples. — twotwotwo
– twotwotwo, Commented Jul 4, 2016 at 6:51

twotwotwo · Accepted Answer · 2016-07-04 07:00:25Z

I think the problem is that an extra reflect.ValueOf meant that you had a reflect.Value referring to another reflect.Value, not to the NullString. The way Printf formatted it kind of obscured that. It looks like values.Field(i) returns the reflect.Value you need. Here's your program minimally modified by taking out the ValueOf:

package main

import (
    "database/sql"
    "fmt"
    "reflect"
)

type fooT struct {
    ID    int64
    Foo   string
    NullS sql.NullString
    FooQ  sql.NullString
}

func main() {
    foo := fooT{
        ID:    42,
        NullS: sql.NullString{"Your mother was a hamster", true},
    }

    types := reflect.TypeOf(foo)
    values := reflect.ValueOf(foo)

    changed := ""
    for i := 0; i < types.NumField(); i++ {
        fieldType := types.Field(i)
        field := values.Field(i)

        switch field.Type().Kind() {
        // Works
        case reflect.String:
            if field.String() != "" {
                changed += fmt.Sprintf("<strong>%s</strong>: %v<br>\n",
                    fieldType.Name, field.String())
            }
        default:
            switch field.Type().String() {
            case "sql.NullString":
        iface := field.Interface()
                fmt.Printf("%s: %T -> %#v\n",
                    fieldType.Name, iface, iface)
            }
        }
    }

    fmt.Printf(changed)
}

You may be able to simplify more by using a type switch to do most of the work, like so:

package main

import (
    "database/sql"
    "fmt"
    "reflect"
)

type fooT struct {
    ID    int64
    Foo   string
    NullS sql.NullString
    FooQ  sql.NullString
}

func main() {
    foo := fooT{
        ID:    42,
        NullS: sql.NullString{"Your mother was a hamster", true},
    }

    values := reflect.ValueOf(foo)

    changed := ""
    for i := 0; i < values.NumField(); i++ {
        v := values.Field(i)
        f := v.Interface()
        switch f := f.(type) {
        case string:
            fmt.Println("string:", f)
        case sql.NullString:
            fmt.Println("NullString:", f.Valid, f.String)
        default:
            fmt.Printf("%s: %v\n", v.Type(), f)
        }
    }

    fmt.Printf(changed)
}

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