Unmarshal nested XML value from dynamic XML document

Steffen Wentzel

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Feb 23, 2019, 6:50:40 AM2/23/19

to golang-nuts

Hi all,

although I'm sure this question came up already I wasn't able to find a solution for my issue.

I have this example XML structure:

<outer>
 <req4711response>
   <return>{D4564-4564-456-4564456}</return>
 </req4711response>
</outer>

I want to unmarshal the ID value contained in the <return> XML tags. However this XML structure is dynamic, the embedding XML tag (in this case <req4711response>) changes with every request.

How can I unmarshal the return value universally?

I have setup a little example: https://play.golang.org/p/j4ttdgW4W77

Also I've read this paragraph in the package documentation:

* If the XML element contains a sub-element whose name matches
   a struct field's XMLName tag and the struct field has no
   explicit name tag as per the previous rule, unmarshal maps
   the sub-element to that struct field.

I dont' know what is meant with "XMLName tag" in this context, but I think it might think it might point to a solution?

Thanks for any suggestions and hints,

Steffen

Tamás Gulácsi

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Feb 23, 2019, 2:53:07 PM2/23/19

to golang-nuts

Walk over it usin xml.Decoder.Token, and UnmarshalElement when found a StartToken with a proper Name.Local.

Steffen Wentzel

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Feb 24, 2019, 12:28:37 PM2/24/19

to golang-nuts

Thanks for the response. I was hoping it would be possible with just xml.Unmarshal.

I now have this implementation - any comments on that? (I'm okay with returning the error inline, it's just for logging purposes):

Playground: https://play.golang.org/p/6J8XndWdlv_N

func id2(axl []byte) string {
	dec := xml.NewDecoder(bytes.NewReader(axl))
	for {
		tok, err := dec.Token()
		if err != nil {
			if err == io.EOF {
				return "token not found"
			}
			return fmt.Sprintf("error decoding XML: %v", err)

		}
		if tok, ok := tok.(xml.StartElement); ok {
			if tok.Name.Local == "return" {
				var id string
				err := dec.DecodeElement(&id, &tok)
				if err != nil {
					return fmt.Sprintf("error decoding return element: %v", err)
				}
				return id
			}
		}
	}
}

Sam Whited

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Feb 24, 2019, 1:13:57 PM2/24/19

to 'Константин Иванов' via golang-nuts

Your function appears to return error text, but it will be much more clear what's going on when reading the code if you encode your intent in the return type and return an error instead of a string (which gives me no information about what that string is supposed to represent: is it an error? is it some XML?). fmt.Errorf can be used for this, also just return the io.EOF directly.

—Sam

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Matt Harden

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Feb 24, 2019, 11:44:34 PM2/24/19

to Steffen Wentzel, golang-nuts

You don't have to use an xml.Decoder. You may be able to use the `xml:",any"` tag for this case.

type Response struct {
XMLName xml.Name
Return string `xml:"return"`
}
type Outer struct {
XMLName struct{} `xml:"outer"`
Responses []Response `xml:",any"`
}

https://play.golang.org/p/j4w4-1uuYae

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Steffen Wentzel

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Feb 25, 2019, 1:04:01 AM2/25/19

to golang-nuts

Very nice, `xml:",any"` was doing the trick. I've condensed the function and here's the (currently) final result: https://play.golang.org/p/G1eGw5gtk7C

func id3(axl []byte) string {
	type Return struct {
		Response struct {
			Return string `xml:"return"`
		} `xml:",any"`
	}
	var ret Return
	err := xml.Unmarshal(axl, &ret)
	if err != nil {
		return fmt.Sprintf("unable to unmarshal id: %v", err) // I know I should use a separate error channel
	}
	return ret.Response.Return // might be empty if XML was invalid (no return element)
}

Thanks for all your comments!

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