Golang custom unmarshalling nested JSON

Question

I have a JSON object that looks like this. It contains 3 meals a day for one week, for 21 total entries, since each meal is an individual entry.

{
    "name": "MealPlan 1508620645147",
    "items": [
        {
            "day": 1,
            "mealPlanId": 0,
            "slot": 1,
            "position": 0,
            "type": "RECIPE",
            "value": "{\"id\":869953,\"imageType\":\"jpg\",\"title\":\"Cream Cheese & Fruit Breakfast Pastries\"}"
        },
        {
            "day": 1,
            "mealPlanId": 0,
            "slot": 2,
            "position": 0,
            "type": "RECIPE",
            "value": "{\"id\":537176,\"imageType\":\"jpg\",\"title\":\"Leftover Rice Casserole\"}"
        },
        {
            "day": 1,
            "mealPlanId": 0,
            "slot": 3,
            "position": 0,
            "type": "RECIPE",
            "value": "{\"id\":595927,\"imageType\":\"jpg\",\"title\":\"Spinach and Cheddar Quiche\"}"
        },
        {
            "day": 2,
            "mealPlanId": 0,
            "slot": 1,
            "position": 0,
            "type": "RECIPE",
            "value": "{\"id\":536716,\"imageType\":\"jpg\",\"title\":\"Candied Pecan Waffles\"}"
        },
        {
            "day": 2,
            "mealPlanId": 0,
            "slot": 2,
            "position": 0,
            "type": "RECIPE",
            "value": "{\"id\":893265,\"imageType\":\"jpg\",\"title\":\"Tahini Date Smoothie Bowls\"}"
        },
        {
            "day": 2,
            "mealPlanId": 0,
            "slot": 3,
            "position": 0,
            "type": "RECIPE",
            "value": "{\"id\":512880,\"imageType\":\"jpg\",\"title\":\"Grilled Caprese Salad Sandwich for #SundaySupper\"}"
        },
        {
            "day": 3,
            "mealPlanId": 0,
            "slot": 1,
            "position": 0,
            "type": "RECIPE",
            "value": "{\"id\":648647,\"imageType\":\"jpg\",\"title\":\"Jumbo Blueberry Muffins\"}"
        },
        {
            "day": 3,
            "mealPlanId": 0,
            "slot": 2,
            "position": 0,
            "type": "RECIPE",
            "value": "{\"id\":548554,\"imageType\":\"jpg\",\"title\":\"Brie, Pesto, and Sweet Pepper Grilled Cheese\"}"
        },
        {
            "day": 3,
            "mealPlanId": 0,
            "slot": 3,
            "position": 0,
            "type": "RECIPE",
            "value": "{\"id\":438024,\"imageType\":\"jpg\",\"title\":\"Mother's Manicotti\"}"
        },
        {
            "day": 4,
            "mealPlanId": 0,
            "slot": 1,
            "position": 0,
            "type": "RECIPE",
            "value": "{\"id\":681594,\"imageType\":\"jpg\",\"title\":\"Huevos Rancheros\"}"
        },
        {
            "day": 4,
            "mealPlanId": 0,
            "slot": 2,
            "position": 0,
            "type": "RECIPE",
            "value": "{\"id\":99184,\"imageType\":\"jpg\",\"title\":\"Black Bean Tacos\"}"
        },
        {
            "day": 4,
            "mealPlanId": 0,
            "slot": 3,
            "position": 0,
            "type": "RECIPE",
            "value": "{\"id\":604514,\"imageType\":\"jpg\",\"title\":\"Cheddar Scallion Dutch Baby\"}"
        },
        {
            "day": 5,
            "mealPlanId": 0,
            "slot": 1,
            "position": 0,
            "type": "RECIPE",
            "value": "{\"id\":622672,\"imageType\":\"jpg\",\"title\":\"Cinnamon-Sugar Streusel Baked French Toast\"}"
        },
        {
            "day": 5,
            "mealPlanId": 0,
            "slot": 2,
            "position": 0,
            "type": "RECIPE",
            "value": "{\"id\":636178,\"imageType\":\"jpg\",\"title\":\"Broccoli Cheddar Soup, A Panera Bread Co. Copycat\"}"
        },
        {
            "day": 5,
            "mealPlanId": 0,
            "slot": 3,
            "position": 0,
            "type": "RECIPE",
            "value": "{\"id\":452482,\"imageType\":\"jpg\",\"title\":\"Slow Cooker Macaroni and Cheese I\"}"
        },
        {
            "day": 6,
            "mealPlanId": 0,
            "slot": 1,
            "position": 0,
            "type": "RECIPE",
            "value": "{\"id\":157272,\"imageType\":\"jpg\",\"title\":\"Pomegranate-Nutella Waffles\"}"
        },
        {
            "day": 6,
            "mealPlanId": 0,
            "slot": 2,
            "position": 0,
            "type": "RECIPE",
            "value": "{\"id\":619111,\"imageType\":\"jpg\",\"title\":\"Barley, Bulgur and Vegetable Vegan Casserole\"}"
        },
        {
            "day": 6,
            "mealPlanId": 0,
            "slot": 3,
            "position": 0,
            "type": "RECIPE",
            "value": "{\"id\":510089,\"imageType\":\"jpg\",\"title\":\"Stovetop Mac and Cheese\"}"
        },
        {
            "day": 7,
            "mealPlanId": 0,
            "slot": 1,
            "position": 0,
            "type": "RECIPE",
            "value": "{\"id\":551869,\"imageType\":\"jpg\",\"title\":\"Berry Smoothie\"}"
        },
        {
            "day": 7,
            "mealPlanId": 0,
            "slot": 2,
            "position": 0,
            "type": "RECIPE",
            "value": "{\"id\":590452,\"imageType\":\"jpg\",\"title\":\"Cheesy Baked Pasta with Eggplant and Artichokes\"}"
        },
        {
            "day": 7,
            "mealPlanId": 0,
            "slot": 3,
            "position": 0,
            "type": "RECIPE",
            "value": "{\"id\":590452,\"imageType\":\"jpg\",\"title\":\"Cheesy Baked Pasta with Eggplant and Artichokes\"}"
        }
    ]
}

I want to unmarshal it into an array of structs that will hold 3 meals each as Breakfast, Lunch, Dinner. So, I want my struct to look like this, where ID is the value.id field from the JSON, and Name is the value.title field from the JSON, Breakfast is the item with slot:1, Lunch is the item with slot:2 and Dinner is the item with slot:3.

type Day struct {
    Breakfast meal
    Lunch meal
    Dinner meal
}

type meal struct {
    ID   int
    Name string
}

How can I accomplish this in Go? My initial thought was to create an intermediate struct that holds all the data from the JSON, and then create another struct by using just the fields I need. How can I do this without using the intermediary struct ?

What about implementing your own implementation of UnmarshalJSON on your primary struct? — mayo
– mayo, Commented Oct 21, 2017 at 22:25
You can write how to read and parse a json file into an structure... check this post: gregtrowbridge.com/golang-json-serialization-with-interfaces — mayo
– mayo, Commented Oct 21, 2017 at 23:35
You already know the answer: use an intermediary struct. There probably is no serious argument against it. Speed will not matter, otherwise you would not be using JSON in the first place. The code for this struct plus the conversion will be about the same size and complexity as writing your own UnmarshalJSON. So just use the straight forward way with a helper struct, this will make it easier to read in the end, which is what is important. — gonutz
– gonutz, Commented Oct 23, 2017 at 7:55
I agree with gonutz, you can write your own version of UnmarshallJSON but you will mix logic there, I mean 1.parsing from json and 2. making the conversion to your desired structure. A better approach is having a helper function or change the json structure or having the extra structure. the question is, if you are not familiar with the code how long it will take to understand, fix, read the code /logic. — mayo
– mayo, Commented Oct 28, 2017 at 17:49

Nishant Roy · Accepted Answer · 2017-10-23 22:52:43Z

Did as mayo suggested and implemented UnmarshalJSON

func (wp *WeekPlan) UnmarshalJSON(b []byte) error {
    wp.Days = make([]Day, 7)
    var f map[string]*json.RawMessage
    json.Unmarshal(b, &f)

    var v []map[string]interface{}
    json.Unmarshal(*f["items"], &v)

    for _, item := range v {

        day := int(item["day"].(float64)) - 1
        mealnumber := int(item["slot"].(float64))

        var value map[string]interface{}
        json.Unmarshal([]byte(item["value"].(string)), &value)

        fmt.Println(value)

        id := int(value["id"].(float64))
        name := value["title"].(string)

        thisMeal := Meal{ID: id, Name: name}

        var dateUpdate Day

        dateUpdate = wp.Days[day]

        switch mealnumber {
        case 1:
            dateUpdate.Breakfast = thisMeal
        case 2:
            dateUpdate.Lunch = thisMeal
        default:
            dateUpdate.Dinner = thisMeal
        }

        wp.Days[day] = dateUpdate
    }

    return nil
}


type WeekPlan struct {
    Days []Day
}


type Day struct {
    Breakfast Meal
    Lunch     Meal
    Dinner    Meal
}


type Meal struct {
    ID       int
    Name     string
    CookTime int
    Image    string
}

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