Dynamically creating nested maps in golang

Question

I'm trying to interpret JSONs in Golang. I'm searching for a specific attribute in an unknown JSON which I know the keys for but my JSON might be really nested. I do know how many 'layers' there are in my JSON.

For example, if my JSON is:

nestedJson = { key1: { key2: { key3: "Found data" } } }

my keys are ["key1", "key2", "key3"] and there are 3 layers in this JSON so I can get the data back out by doing

var nestedJson = []map[string]map[string]map[string]interface{}
json.Unmarshal([]byte(nestedJon), &nestedJson)
data := nestedJson["key1]["key2"]["key3"]

What I want to be able to do is dynamically create that nestedJson object by specifying an integer value for the number of layers is should search through.

Can anyone help with this? Please let me know if I haven't been clear enough!

Eli Bendersky · Accepted Answer · 2019-01-26 15:23:02Z

In general, for cases like this you'll have to use the generic unmarshaling capability of json.Unmarshal into an interface{}.

b := []byte(`{ "key1": { "key2": { "key3": "Found data" } } } `)
var f interface{}
if err := json.Unmarshal(b, &f); err != nil {
    panic(err)
}
fmt.Println(f)

And now you can use a bunch of type assertions to explore f, for example to find which keys it contains:

m := f.(map[string]interface{})
for k, v := range m {
    if k == "key3" {
        fmt.Println("found key3. maps to", v)
    }
}

If you don't find key3 at this level, use recursion to check all the values - are they maps with key3 keys, and so on... Something like

func findNested(m map[string]interface{}, s string) (bool, interface{}) {
    // Try to find key s at this level
    for k, v := range m {
        if k == s {
            return true, v
        }
    }
    // Not found on this level, so try to find it nested
    for _, v := range m {
        nm := v.(map[string]interface{})
        found, val := findNested(nm, s)
        if found {
            return found, val
        }
    }
    // Not found recursively
    return false, nil
}

Note: this function was quickly hacked together and probably mishandles a bunch of corner cases. It's here to demonstrate the key idea - use it as a basis for your specific needs

Juan-Kabbali · Accepted Answer · 2021-03-05 22:50:39Z

Personally i love to use gabs module which allows to handle those kind of situation in a more human friendly way.

For installing the module use:

go get github.com/Jeffail/gabs/v2

Handy usage examples

// jsonParsed var contains a set of functions to play arround
jsonParsed, _ := gabs.ParseJSON([]byte(`{
    "outter":{
        "inner":{
            "value1":10,
            "value2":22
        },
        "alsoInner":{
            "value1":20,
            "array1":[
                30, 40
            ]
        }
    }
}`))

// for your case, it's useful Exists or ExistsP functions
exists := jsonParsed.Exists("outter", "inner", "value1")
// exists == true

exists = jsonParsed.ExistsP("outter.inner.value3")
// exists == false

As you need some of dynamic key search, you can use ChildrenMap function to iterate and validate the keys existence with the previous explained functions.

jsonParsed, err := gabs.ParseJSON([]byte(`{"object":{"first":1,"second":2,"third":3}}`))
if err != nil {
    panic(err)
}

// S is shorthand for Search
for key, child := range jsonParsed.S("object").ChildrenMap() {
    fmt.Printf("key: %v, value: %v\n", key, child.Data().(float64))
    // here you can use Exists or ExistsP
}
// key: first, value: 1
// key: second, value: 2
// key: third, value: 3

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