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 ?

Answer 1

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
}