How to convert *sql.Rows to typed JSON in Golang

Question

Essentially, I am trying to run a query on a MySQL database, get the data made converted into JSON and sent back to the client. I have tried several methods and all of the "easy" ones result in sending back all of the JSON as a string. I need this to be send back as a key (string) with []float64 value. This way I have an array of data associated with a key. Also, this needs to have a type. The best method I've found so far to accomplish this was to build put all of the data into a struct, encode it and send that back to the ResponseWriter.

I have seen several questions on making JSON from a database but I haven't found anything utilizing the struct method. I wrote the below code into a single function to illustrate my question. This is VERY limited in that it will only handle two fields and it MUST be a float64.

Therefore, my question is: How do I create this JSON from a query response that has the correct type before sending this back to the client and is there a way to do this dynamically (ie, can accept a variable number of columns and unknown types)?:

{ "Values":[12.54, 76.98, 34.90], "Dates": ["2017-02-03", "2017-02-04:, "2017-02-05"]}

    type DbDao struct{
      db *sql.DB
    }

    type JSONData struct {
      Values []float64
      Dates []string
    }


    func (d *DbDao) SendJSON(sqlString string, w http.ResponseWriter) (error) {

      stmt, err := d.db.Prepare(sqlString)
      if err != nil {
        return err
      }
      defer stmt.Close()

      rows, err := stmt.Query()
      if err != nil {
        return err
      }
      defer rows.Close()

      values := make([]interface{}, 2)
      scanArgs := make([]interface{}, 2)
      for i := range values {
        scanArgs[i] = &values[i]
      }

      for rows.Next() {
        err := rows.Scan(scanArgs...)
        if err != nil {
          return err
        }

        var tempDate string 
        var tempValue float64
        var myjson JSONData

        d, dok := values[0].([]byte)
        v, vok := values[1].(float64)

        if dok {
          tempDate = string(d)
          if err != nil {
            return err
          }
          myjson.Dates = append(myjson.Dates, tempDate)
        }

        if vok {      
          tempValue = v 
          myjson.Values = append(myjson.Values, tempValue)
          fmt.Println(v)
          fmt.Println(tempValue)

        }    

        err = json.NewEncoder(w).Encode(&myjson)
        if err != nil {
          return err
        }
      }

      return nil 
    }

Answer 1

This is the best implementation that I was able to come up with that would make it dynamic. It is also significantly shorter than my original. As I've seen this type of question quite a bit, I hope this helps others. I am open to other answers that have a better implementation of this:

func (d *DbDao) makeStructJSON(queryText string, w http.ResponseWriter) error {

    // returns rows *sql.Rows
    rows, err := d.db.Query(queryText)
    if err != nil {
        return err
    }
    columns, err := rows.Columns()
    if err != nil {
        return err
    }

    count := len(columns)
    values := make([]interface{}, count)
    scanArgs := make([]interface{}, count)
    for i := range values {
        scanArgs[i] = &values[i]
    }

    masterData := make(map[string][]interface{})

    for rows.Next() {
        err := rows.Scan(scanArgs...)
        if err != nil {
            return err
        }
        for i, v := range values {

            x := v.([]byte)

            //NOTE: FROM THE GO BLOG: JSON and GO - 25 Jan 2011:
            // The json package uses map[string]interface{} and []interface{} values to store arbitrary JSON objects and arrays; it will happily unmarshal any valid JSON blob into a plain interface{} value. The default concrete Go types are:
            //
            // bool for JSON booleans,
            // float64 for JSON numbers,
            // string for JSON strings, and
            // nil for JSON null.

            if nx, ok := strconv.ParseFloat(string(x), 64); ok == nil {
                masterData[columns[i]] = append(masterData[columns[i]], nx)
            } else if b, ok := strconv.ParseBool(string(x)); ok == nil {
                masterData[columns[i]] = append(masterData[columns[i]], b)
            } else if "string" == fmt.Sprintf("%T", string(x)) {
                masterData[columns[i]] = append(masterData[columns[i]], string(x))
            } else {
                fmt.Printf("Failed on if for type %T of %v\n", x, x)
            }

        }
    }

    w.Header().Set("Content-Type", "application/json")

    err = json.NewEncoder(w).Encode(masterData)

    if err != nil {
        return err
    }
    return err
}