How to get file posted from JSON in go gin?

Question

I want to save image file posted by JSON.

Here is the struct of the post:

type Article struct {
    Title   string `json:"title"`
    Body string `json:"body"`
    File    []byte `json:"file"`
}

And the handler is:

   func PostHandler(c *gin.Context) {
        var err error
        var json Article
        err = c.BindJSON(&json)
        if err != nil {
            log.Panic(err)
        }

    //handle photo upload
        var filename string
        file, header, err := json.File  //assignment count mismatch: 3 = 1

        if err != nil {
            fmt.Println(err)
            filename = ""

        } else {
            data, err := ioutil.ReadAll(file)
            if err != nil {
                fmt.Println(err)
                return
            }

            filename = path.Join("media", +shared.RandString(5)+path.Ext(header.Filename))

            err = ioutil.WriteFile(filename, data, 0777)
            if err != nil {
                io.WriteString(w, err.Error())
                return
            }

        }
...

But I get

assignment count mismatch: 3 = 1

I copied the file handling part from a working multipart form handler which worked fine but apparently,

file, header, err := r.FormFile("uploadfile")

can not be translated into JSON handling.

I have looked at gin docs but could not find examples involving json file handling. So how can I fix this?

Answer 1

Using Gin to get an uploaded file

I think your question is "Using Gin, how do I get an uploaded file?". Most developers don't upload files with JSON encoding, which could be done but requires the file to be included as a base64 string (and increases the file size about 33%).

The common (and more efficient) practice is to upload the file using the "multipart/form-data" encoding. The code that others provided file, header, err:= c.Request.FormFile("file") works, but that hijacks the underlining "net/http" package that Gin extends.

My recommendation is to use ShouldBind , but you can also use the FormFile orMultipartForm methods provided by the Gin package.

Examples below. Similar (but less detailed) explanations are also offered on the Gin page https://github.com/gin-gonic/gin#model-binding-and-validation and https://github.com/gin-gonic/gin#upload-files .

---

Upload one file

---

Clients

HTML

<form action="/upload" method="POST">
  <input type="file" name="file">
  <input type="submit">
</form>

Curl

curl -X POST http://localhost:8080/upload \
  -F "file=../path-to-file/file.zip" \
  -H "Content-Type: multipart/form-data"

Server

Go

package main

import (
    "github.com/gin-gonic/gin"
    "log"
    "net/http"
    "io/ioutil"
)

type Form struct {
    File *multipart.FileHeader `form:"file" binding:"required"`
}

func main() {
    router := gin.Default()

    // Set a lower memory limit for multipart forms (default is 32 MiB)
    // router.MaxMultipartMemory = 8 << 20  // 8 MiB

    router.POST("/upload", func(c *gin.Context) {

        // Using `ShouldBind`
        // --------------------
        var form Form
        _ := c.ShouldBind(&form)

        // Get raw file bytes - no reader method
        // openedFile, _ := form.File.Open()
        // file, _ := ioutil.ReadAll(openedFile)

        // Upload to disk
        // `form.File` has io.reader method
        // c.SaveUploadedFile(form.File, path)
        // --------------------

        // Using `FormFile`
        // --------------------
        // formFile, _ := c.FormFile("file")

        // Get raw file bytes - no reader method
        // openedFile, _ := formFile.Open()
        // file, _ := ioutil.ReadAll(openedFile)

        // Upload to disk
        // `formFile` has io.reader method
        // c.SaveUploadedFile(formFile, path)
        // --------------------
        
        c.String(http.StatusOK, "Files uploaded")
    })

    // Listen and serve on 0.0.0.0:8080
    router.Run(":8080")
}

---

Upload multiple files

---

Clients

HTML

<form action="/upload" method="POST" multiple="multiple">
  <input type="file" name="files">
  <input type="submit">
</form>

Curl

curl -X POST http://localhost:8080/upload \
  -F "files=../path-to-file/file1.zip" \
  -F "files=../path-to-file/file2.zip" \
  -H "Content-Type: multipart/form-data"

Server

Go

package main

import (
    "github.com/gin-gonic/gin"
    "log"
    "net/http"
    "io/ioutil"
)

type Form struct {
    Files []*multipart.FileHeader `form:"files" binding:"required"`
}

func main() {
    router := gin.Default()

    // Set a lower memory limit for multipart forms (default is 32 MiB)
    // router.MaxMultipartMemory = 8 << 20  // 8 MiB

    router.POST("/upload", func(c *gin.Context) {

        // Using `ShouldBind`
        // --------------------
        var form Form
        _ := c.ShouldBind(&form)

        // for _, formFile := range form.Files {

          // Get raw file bytes - no reader method
          // openedFile, _ := formFile.Open()
          // file, _ := ioutil.ReadAll(openedFile)

          // Upload to disk
          // `formFile` has io.reader method
          // c.SaveUploadedFile(formFile, path)

        // }
        // --------------------

        // Using `MultipartForm`
        // --------------------
        // form, _ := c.MultipartForm()
        // formFiles, _ := form["files[]"]

        // for _, formFile := range formFiles {

          // Get raw file bytes - no reader method
          // openedFile, _ := formFile.Open()
          // file, _ := ioutil.ReadAll(openedFile)

          // Upload to disk
          // `formFile` has io.reader method
          // c.SaveUploadedFile(formFile, path)

        // }
        // --------------------
        
        c.String(http.StatusOK, "Files uploaded")
    })

    // Listen and serve on 0.0.0.0:8080
    router.Run(":8080")
}

Answer 2

In your code you say var json Article where type article is defined as

type Article struct {
    Title   string `json:"title"`
    Body string `json:"body"`
    File    []byte `json:"file"`
}

And File is of type []byte . Type byte doesn't return anything other than what it holds

Your Article.File is not the same as r.FormFile where FormFile is a method that returns 3 items

So file, header, err:= json.File isn't file, header, err:= r.FormFile("foo")

See the implementation and method description from godocs -> here