JSON Unmarshal when structure is determined at runtime

Question

Usually we Unmarshal a json object in Go as:

I am kind of a noob in Go, so pardon me if some syntax seems incorrect below.

type Item struct {
    Quantity    int     `json:"quantity,omitempty"`
    Price       float64 `json:"price,omitempty"`
    Currency    string  `json:"currency,omitempty"`
}
output := &Item{}
err:= json.Unmarshal([]byte(data), output)

Now the catch is my json could be different at runtime depending on some field. Price could be string, array with different value or json containing currency and price in one object.

I have this mapping in the db, how could I write my code so that I would read that mapping of column name to type and then unmarshall it creating the suitable struct at runtime. For e.g. I need to unmarshall following JSON's in same code:-

{"Quantity": 5, "Price": 64.5, "Currency": "USD"}
{"Quantity": 5, "Purchase": {"Price": 64.5, "Currency": "USD"}}

I would already have mapping like Quantity -> int, Purchase -> JSON String for second json with me in the DB.

tl;dr

Need to unmarshall json where structure changes at runtime on the basis of some parameters and I know the structure beforhand

Edit: Rephrase

I need function which will return me the object of above struct take json string and json format string as input.

CustomUnmarshal([]byte(data) []byte, format string) (*item){}

I have written a sample code here:-

https://play.golang.org/p/JadYOnBQne

Are the listed two cases your only possible inputs ? Or are there other possible forms ? `{"Quantity": 5, "Price": 64.5, "Currency": "USD"} {"Quantity": 5, "Purchase": {"Price": 64.5, "Currency": "USD"}}` — John S Perayil, Oct 20 '16 at 06:45
@JohnSPerayil There are other possible formats. That is the reason I want to write something generic rather than writing code everytime, we have a new integration. — Mangat Rai Modi, Oct 20 '16 at 07:04
Now when I think about it, I think it is not possible. After unmarshalling, the function has to return something of a given defined type. Let it be a void interface, but again I have to manually map it to data in Item struct — Mangat Rai Modi, Oct 20 '16 at 07:08
http://stackoverflow.com/questions/32428797/unmarshal-to-a-interface-type — Jiang YD, Oct 20 '16 at 08:08
@MangatRaiModi Hi, I was cleaning up old answers, let me know if my answer was useful. If not I would prefer to delete it. Thank You — John S Perayil, Nov 17 '16 at 06:49
@JohnSPerayil I totally forgot to update. Btw, I believe SO has very strict policy against deleting the answers. You might want to look into that. — Mangat Rai Modi, Nov 17 '16 at 06:57
I didn't see anything against deleting per say, I just wanted to remove answers that others didn't find useful. There is a daily limit to deletions though. — John S Perayil, Nov 17 '16 at 07:21

John S Perayil · Accepted Answer · 2016-10-20T15:03:08.383

If your output structure and the keys in the input remain the same then it is possible to do what you require using the Unmarshaler interface.

type Unmarshaler interface {
        UnmarshalJSON([]byte) error
}

I've implemented a very crude, string only struct implimentation.

type Item struct {
    Quantity string `json:"quantity,omitempty"`
    Price    string `json:"price,omitempty"`
    Currency string `json:"currency,omitempty"`
}

Like I said it's very crude with a lot of assumptions and no checks in place.

func (itm *Item) UnmarshalJSON(byt []byte) (err error) {
    jsn := string(byt)
    jsn = strings.Replace(jsn,"{","",-1)
    jsn = strings.Replace(jsn,"}","",-1)

    jsnArr := strings.Split(jsn,":")
    fmt.Println(jsnArr)

    for i, val := range jsnArr {
        switch {
        case strings.Contains(val,"Quantity"):
            itm.Quantity = strings.Split(jsnArr[i+1],",")[0]
        case strings.Contains(val,"Price"):
            itm.Price = strings.Split(jsnArr[i+1],",")[0]
        case strings.Contains(val,"Currency"):
            itm.Currency = strings.Split(jsnArr[i+1],",")[0]    
        }
    }
    return
}

Full Program

Also note that the json tags you set for the structure represents how the json input keys should be, so keys like "Quantity" should be "quantity" in the JSON input, and so on.

So string parsing was the only solution. Thanks for the answer. — Mangat Rai Modi, Nov 17 '16 at 06:56

JSON Unmarshal when structure is determined at runtime

1 Answers1