通过反射获取sql.NullString的值

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
", iface, iface)
    fmt.Printf("%T -> %#v -> %#v
", 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>
",
                    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
",
                    fieldType.Name, iface, iface)

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

    fmt.Printf(changed)
}
dongzen2675
dongzen2675 感谢您提供完整的程序。我认为您在NullString情况下不需要那个reflect.ValueOf(请注意,在字符串情况下则不需要)。我可能会误解这个问题,但是我将用几个代码示例发布答案。
大约 4 年之前 回复

1个回答

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>
",
                    fieldType.Name, field.String())
            }
        default:
            switch field.Type().String() {
            case "sql.NullString":
        iface := field.Interface()
                fmt.Printf("%s: %T -> %#v
",
                    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
", v.Type(), f)
        }
    }

    fmt.Printf(changed)
}
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