FormValue is always an empty map

Question

I programmed multiple methods for my handler which look like this for example:

func DeleteProduct(w http.ResponseWriter, r *http.Request){
    log.Println(r.Form)
    db.Exec("Delete from  products where Id = "+r.FormValue("Id"))
}

The problem is that r.Form is always an empty map, in my delete request I send the Id in a JSON which looks like this:

    {
        "CustomerDate": "13.03.2018",
        "CustomerDateTime": "13:30",
        "UserId": 4
    }

in the main method I register the handler methods like this:

        router.HandleFunc("/delete",handler.DeleteProduct).Methods("DELETE")

Why is the r.Form and the r.PostForm always an empty map?

https://golang.org/pkg/net/http/#Request.Form specifically this part *"This field is only available after ParseForm is called."* and btw `FormValue` should call `ParseForm` if it hasn't yet been. — mkopriva, May 16 '18 at 12:43
Secondly, and more importantly Form does not contain json data. Form, after parsing, will hold data sent as `application/x-www-form-urlencoded` + data sent as query parameters, if no such data was sent then Form will be empty even after parse. — mkopriva, May 16 '18 at 12:45
See here on how to get the json from a request https://stackoverflow.com/a/15685432/965900 — mkopriva, May 16 '18 at 12:46
Not related to your question, but you're opening yourself up to a huge SQL injection attack this way. PLEASE use placeholders in your SQL queries instead of building a query with string concatenation. — Jonathan Hall, May 16 '18 at 12:50
As @mkopriva states you require a call to `r.ParseForm ()` in order to populate data structures with actual values. From other side it would be good to see the client code in order to verify that your are sending properly the url params or body content — Victor, May 16 '18 at 12:50

cn007b · Accepted Answer · 2018-05-16T13:24:11.797

3

In case of JSON request you have to unmarshal request body before using any parameter.
For example:

type ReqBody struct {
  CustomerDateTime string `json:"CustomerDateTime"`
  CustomerDateTime string `json:"CustomerDateTime"`
  UserId           int    `json:"UserId"`
}

body, err := ioutil.ReadAll(req.Body)
if err != nil {
    // Error handler...
}
rb := ReqBody{}
json.Unmarshal(body, &rb)
// Now you can perform something like this:
println(rb.UserId)

edited May 16 '18 at 13:24

answered May 16 '18 at 12:51

cn007b

16,596
7
59
74

thank you for the fast answer, I understand what you are trying to tell me but how can I access the body? – Johannes Gnadlinger May 16 '18 at 13:09
@JohannesGnadlinger You can use `req.Body` where `req is req *http.Request`. – cn007b May 16 '18 at 13:18
yeah I already thought this would be the solution but I need a byte[] to unmarhal and not an io.ReadCloser – Johannes Gnadlinger May 16 '18 at 13:21
@JohannesGnadlinger Also you have to do: `body, err := ioutil.ReadAll(req.Body)` – cn007b May 16 '18 at 13:22
awesome thank you for your help now it finally works i am new to this language and you really helped me here :) – Johannes Gnadlinger May 16 '18 at 13:28
@JohannesGnadlinger Happy to help!) – cn007b May 16 '18 at 13:34
@JohannesGnadlinger just FYI you don't need `[]byte` to unmarshal the request's body, you can use a `json.Decoder`, passing the body to its constructor func and then calling `Decode`. This is, AFAICT, the preferred method to unmarshal json requests. – mkopriva May 16 '18 at 13:42
Yes, as another option it's possible to use: `json.NewDecoder(req.Body).Decode(&rb)`. Thank you @mkopriva ! – cn007b May 16 '18 at 13:50

FormValue is always an empty map

1 Answers1