Unmarshal XML element to interface{}

[hajd...@gmail.com]

Is there any reason this *shouldn't* work? ( I realize it doesn't, just curious since you can marshal an empty interface ).

A contrived example: ( https://play.golang.org/p/s-Lh0qDwAP)

type Person struct {

XMLName   xml.Name    `xml:"person"`

FirstName string      `xml:"firstname"`

Parents interface{} `xml:"parent"`

}

func main() {

type Parent struct {

FirstName string `xml:"name"`

LastName string `xml:"lastname"`

}

p := &Person{FirstName: "Steve"}

p.Parents = []Parent{Parent{"Jack","Doe"}, Parent{"Lynne","Doe"}}

data, err := xml.Marshal(p)

if err != nil {

fmt.Println(err)

}

fmt.Println("Marshalled:",string(data))

p2 := &Person{}

p2.Parents = []Parent{}

err = xml.Unmarshal(data, p2)

if err != nil {

fmt.Println(err)

}

fmt.Println("Unmarshalled:", p2)

}

I get that by being an empty interface{}, there are no fields that are exported for it to look at.  Except that I'm assigning a concrete type to the interface{}, so shouldn't Unmarshal be able to reflect that interface{} and find the struct/tag names to Unmarshal the data?  I'm assuming ( and guessing I'm wrong ) that that's how Marshal does it?  I think I'd probably be less confused if Marshal wasn't working as well, but I don't get why Marshal can deal with a interface{} and Unmarshal can't.

The real use case I have being that the XML response I'm getting has some static elements and then it has dynamic elements:

<api>

  <staticfield1>data</staticfield1>

  <staticfield2>data</staticfield2>

  <record>

    <fieldname1>data</fieldname1>

    <fieldname2>data</fieldname2>

  </record>

  <record>

    ..

  </record>

</api>

I have the static fields in a struct, but fieldname's returned in the record elements change, depending on which database table is being returned by the api.  I want to be able to create a record struct that matches what I know the record will look like for each call, and then compose the whole struct ( static fields + the dynamic fields ) that I'm unmarshalling into.

The solution I have right now is to have a byte array that captures the innerxml and then unmarshal that separately, but I felt like this would be a cleaner solution.  ( https://play.golang.org/p/z3BS6tjqgW )

If there isn't anyway to do this - is there anyway to grab a whole <parent> record, with the <parent></parent> tags included?  My byte solution works, but the issue I run into is that my innerxml field just ends up with <FirstName></FirstName><LastName></LastName>, with no surrounding <parent> tags.  So I have to add the parent tags back so that I can unmarshal to a Parent struct, as you can see in the example.

Any help appreciated!

Steve

[Matt Harden]

Here's one approach. https://play.golang.org/p/2DVnkpC6nj

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[hajd...@gmail.com]

Thanks Matt,

I had already tried doing it that way and had run into some issues.  After your post, I tried it again and I must have had something else wrong before, because it worked this time.

I do wish there was still a way to allow interface{} to be Unmarshalled.  It'd be nice to have a function that could compose the struct, rather than forcing the end user of my library to make their own composed struct and pass it along to the library call.

[Matt Harden]

You can do that by implementing your own UnmarshalXML method: https://play.golang.org/p/uoim__Qpeu. You have to make sure to store a pointer to a slice in the interface or else Unmarshal will produce an error. You could only get around *that* limitation by using reflection.

[luiz....@gmail.com]

Dude, thanks a lot! Really really really a lot.

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[Saied Seghatoleslami]

I have tried unmarshalling various JSON and XML files with limited success.  So I ended up writing a "lexer" along the lines of Rob Pike's lecture in Australia.  I have had good success with it in a bunch of projects.  I would be very interested to get some comments on what the community thinks of decoding XML, JSON and similarly encoded objects in this way.  https://github.com/Saied74/lexer2

[Muse254]

@[matt....@gmail.com] deserves a beer, I really needed to unmarshal to an interface