Write JSON of a struct to an http.ResponseWriter

[Frank Blechschmidt]

Hey! :)

I try to implement a simple json api for http://datatables.net

For example I have a struct like that: Products {1 2 2 [{1 Malm 4 Pieces 258.7 20 258.7} {2 Brimnes 2 Pieces 180.7 10 180.7}]}

It has 3 ints and an array of structs of type ProductForSalesorder

type ProductJSON struct {

    sEcho  string

    iTotalRecords   int

    iTotalDisplayRecords int

    aaData []model.ProductForSalesorder

}

type ProductForSalesorder struct {

  ID int64

  Name string

  Quantity int64

  Unit string

  Price float64

  Shippingcost float64

  Promisedate string

}

How could i write to w http.ResponseWriter as JSON?

I tried:

jsonanswer, _ := json.Marshal(products)

fmt.Fprintf(w, "%s", jsonanswer)

and

enc := json.NewEncoder(w)

enc.Encode(products)

Nothing works for me so far :(

Best regards

Frank

[jorelli]

the second example should work fine.  Why are you discarding your error values?  You might want to look at those.  They typically contain error information.

and what's the output on the http response?  nothing?

[Frank Blechschmidt]

the http response:

{}

not really great :(

enc.Encode(products) givs me a nil error

[Jeremy Wall]

Which struct are you encoding? And how are you creating the products value?

> --
>
>

[Frank Blechschmidt]

I have a struct called ProductJSON:

type ProductJSON struct {

    sEcho  string

    iTotalRecords   int

    iTotalDisplayRecords int

    aaData []model.ProductForSalesorder

}

which includes an array of a struct []ProductForSalesorder

type ProductForSalesorder struct {

  ID int64

  Name string

  Quantity int64

  Unit string

  Price float64

  Shippingcost float64

  Promisedate string

}

So my struct could look like: {1 2 2 [{1 Malm 4 Pieces 258.7 20 258.7} {2 Brimnes 2 Pieces 180.7 10 180.7}]}

An this struct should be converted into json for my responsewriter.

[Frank Blechschmidt]

Thats what I try to achieve: http://datatables.net/release-datatables/examples/data_sources/server_side.html

[Cole Mickens]

My guess is, the encoding works fine and it's encoding exactly what you've told it to. The `json` package can't see any fields of ProductJSON as ProductJSON does not export any of its fields. (Hint, try capitalizing sEcho, iTotal{Display,}Records, aaData).

[Frank Blechschmidt]

Thanks, capitalizing works:

{"SEcho":"1","ITotalRecords":2,"ITotalDisplayRecords":2,"AaData":[{"ID":1,"Name":"Malm","Quantity":4,"Unit":"Pieces","Price":258.7,"Shippingcost":20,"Promisedate":"258.7"},{"ID":2,"Name":"Brimnes","Quantity":2,"Unit":"Pieces","Price":180.7,"Shippingcost":10,"Promisedate":"180.7"}]}

But I need the first char of SEcho, ITotalRecords, ITotalDisplayRecords and AaData in lowercase and 

"AaData": [{"ID":1,"Name":"Malm","Quantity":4,"Unit":"Pieces","Price":258.7,"Shippingcost":20,"Promisedate":"258.7"},{"ID":2,"Name":"Brimnes","Quantity":2,"Unit":"Pieces","Price":180.7,"Shippingcost":10,"Promisedate":"180.7"}]}
should look like 
  "aaData": [
    [
      "1",
      "Malm",
      "4",
      "Pieces",
      "258.7",
            "20",
            "258.7"
    ],
    [
      "2",
      "Brimnes",
      "2",
      "Pieces",
      "180.7",
            "10",
            "180.7"
    ]
]

How to do so?

[Frank Blechschmidt]

Ok, I get the aaData array working, but I need the struct members in lowercase for datatables. How?

[Andy Balholm]

On Wednesday, October 17, 2012 2:16:48 PM UTC-7, Frank Blechschmidt wrote:

Ok, I get the aaData array working, but I need the struct members in lowercase for datatables. How?

 You need to use struct tags:

type ProductJSON struct {

    Echo  string `json:"sEcho"`

    TotalRecords   int `json:"iTotalRecords"`

    TotalDisplayRecords int `json:"iTotalDisplayRecords"`

    Data []model.ProductForSalesorder `json:"aaData"`

}

[Frank Blechschmidt]

Thanks, it's working now. You are the best :D

[Cole Mickens]

Hi Frank,

I'm glad my hint helped. You can always transform that data in javascript if you want as well, but you can also use the struct tags to output it straight out of Go. I tend to put struct tags on all the fields of my structs, but that also might have to do with my OCD regarding lowercase names in my json/javascript.

In the future, if you get stuck on json encoding/decoding, the docs for the json package are quite good: http://golang.org/pkg/encoding/json/ (watch out for the Example >) links, I miss them often when looking for help in the docs.

This blog post is more demonstrative: http://blog.golang.org/2011/01/json-and-go.html

Between those two links, I think your issues here, and ones you may run into soon, are discussed.

Best of luck,

Cole

[Richard Varno]

Thanks for the capitalizing hint!  Totally fixed my issue

[cornel...@gmail.com]

I'm trying to do something similar and I'm stuck on the similar problem. How did you solve the aaData problem so the JSON is formatted properly for DataTables?

On Wednesday, October 17, 2012 2:16:48 PM UTC-7, Frank Blechschmidt wrote:

[Shawn Milochik]

On Sat, Jun 21, 2014 at 7:07 PM, <cornel...@gmail.com> wrote:

I'm trying to do something similar and I'm stuck on the similar problem. How did you solve the aaData problem so the JSON is formatted properly for DataTables?

Use a JSON field tag to specify a different name when you marshal:

https://groups.google.com/d/msg/golang-nuts/sK0YSw7HgP4/aA5DiiiQowgJ 

[cornel...@gmail.com]

Hi Shawn, thanks for the reply. The JSON tags were already tagged, the problem was actually on the DataTables side, not on the Go JSON side. Go figure. 

Anyways, for anyone else who finds the same issues this can be solved in two ways. DataTables by default expects a 2D array so you can build that on the fly (which I find ugly) or just modify you initialization of DataTable() to accept your marshaled data as-is.

var table = $('#example').DataTable( 
    {
        ...
       "sAjaxSource": "/api/allfirmware",
        "columns": [
                {
                "class":          'details-control',
                "orderable":      false,
                "data":           null,
                "defaultContent": ''
            },
            { "data": "make" },
            { "data": "model" },
            { "data": "interface" },
            { "data": "country" },
            { "data": "special" },
            { "data": "revision" }
        ],
    });