Saving pdf in MongoDb using GoLang mgo

Question

I read multiple blogs about saving the file using mgo, but unable to find a solution for below specific need, help shoutout!

Below inserts object in MongoDb:

var dbSchoolPojo dbSchool
i := bson.NewObjectId()
dbSchoolPojo.ID = i
coll := db.C("school")
coll.Insert(&dbSchoolPojo)

Below able to get hold of file:

file, handler, err := r.FormFile("pdf") //Able to get file from r *http.Request

Now, before inserting object, I need to set above file like:

dbSchoolPojo.pdf = file.Bytes(); //Of course Bytes() is invalid method

My structs object:

type dbSchool struct {
    ID  bson.ObjectId  `json:"id" bson:"_id,omitempty"`
    ...
    Pdf  []byte  `json:"pdf" bson:"pdf"`
    ...
}

In layman's term, the question is: How do I insert file (received from HTML form) in mongoDb via GoLang struct using mgo driver?

Thanks for reading! :)

---

Update:

PDF is stored in MongoDB like below:

Binary('EWHKDSH876KJHlkjdswsddsfsd3232432njnjkn2dljDSFSDFIJSFD333...')

Below code works without error, but doesn't serve PDF file:

func DownloadPdf(w http.ResponseWriter, r *http.Request, db mongoDB) {
    var school dbSchool
    coll := db.C("schools")

    incomingId = "59e6404e2f68182a74610f19"; //This mongo DB _id is received from GET URL request

    err := coll.Find(bson.M{"_id": bson.ObjectIdHex(incomingId)}).
        Select(bson.M{"pdf": 1}).One(&school)
    if err != nil {
        serve404(w, r, db)
        return
    }

    buffer := school.Pdf
    w.Header().Set("Content-Disposition", "attachment; filename=abc.pdf")
    w.Header().Set("Content-Type", "application/pdf")
    w.Header().Set("Content-Length", strconv.Itoa(len(buffer)))

    if _, err := w.Write(buffer); err != nil {
        log.Println("unable to serve image.") //This line is not executed
    }
}

JQuery code to request content:

      $(".downloadPdfFile").click(function() {
            var domain = document.location.origin;
            window.open(domain+'/brochure/59e6404e2f68182a74610f19', '_blank');
         });

Answer 1

The file returned by Request.FormFile() is of type multipart.File which is:

type File interface {
    io.Reader
    io.ReaderAt
    io.Seeker
    io.Closer
}

It implements io.Reader, so you can simply read its content e.g. with ioutil.ReadAll():

data, err := io.ReadAll(file)
// Handle error

Then:

dbSchoolPojo.Pdfdata = data

But storing big files as part of documents is not optimal / efficient. Instead take a look at the MongoDB GridFS that is also supported by mgo: Database.GridFS().

Here's how you can store a file in MongoDB GridFS (example taken from GridFS.Create()):

func check(err error) {
    if err != nil {
        panic(err.String())
    }
}
file, err := db.GridFS("fs").Create("myfile.txt")
check(err)
n, err := file.Write([]byte("Hello world!"))
check(err)
err = file.Close()
check(err)
fmt.Printf("%d bytes written\n", n)

Using GridFS, you can also save the file without having to read all its content into memory first, if you "stream" the file content into the mgo.GridFile, as it implements io.Writer. Call io.Copy():

// ...
file, handler, err := r.FormFile("pdfFile")

// ...
mongoFile, err := db.GridFS("fs").Create("somepdf.pdf")
check(err)

// Now stream from HTTP request into the mongo file:
written, err := io.Copy(mongoFile, file)
// Handle err

err = mongoFile.Close()
check(err)

Edit: update answering your update

When you serve the PDF, you query the document in a wrong way:

err := coll.Find(bson.M{"_id": bson.ObjectIdHex(incomingId)}).
    Select(bson.M{"pdf": 1}).One(&school)

You select the pdf field to retrieve, but your documents in MongoDB have no such field:

type dbSchool struct {
    ID  bson.ObjectId  `json:"id" bson:"_id,omitempty"`
    ...
    Pdfdata  []byte  `json:"pdf"`
    ...
}

This type definition will result having a pdfdata field, but not pdf. Add the proper mgo struct tag:

type dbSchool struct {
    ID  bson.ObjectId  `json:"id" bson:"_id,omitempty"`
    ...
    Pdfdata  []byte  `json:"pdf" bson:"pdf"`
    ...
}

Or change the selected field from pdf to pdfdata.