How do I populate and append a nested array in JSON using golang?

Question

I am trying to learn how to create and manipulate JSON in this format on the fly using golang:

{ 
"justanarray": [ 
    "One", 
    "Two" 
], 
"nestedstring": {"name": {"first": "Dave"}}, 
"nestedarray": [ 
    {"address": {"street": "Central"}},  
    {"phone": {"cell": "(012)-345-6789"}}  
] 
}

I can create and manipulate everything but the nested array.

Here is a play for the code below. https://play.golang.org/p/pxKX4IOE8v

package main 

import ( 
        "fmt" 
        "encoding/json" 
) 





//############ Define Structs ################ 

//Top level of json doc 
type JSONDoc struct { 
        JustArray   []string    `json:"justanarray"` 
    NestedString    NestedString    `json:"nestedstring"` 
        NestedArray []NestedArray   `json:"nestedarray"` 


} 

//nested string 
type NestedString struct { 
        Name   Name   `json:"name"` 
} 
type Name struct { 
        First string `json:"first"` 
} 

//Nested array 
type NestedArray []struct { 
        Address   Address   `json:"address,omitempty"` 
        Phone Phone `json:"phone,omitempty"` 
} 
type Address struct { 
        Street string `json:"street"` 
} 
type Phone struct { 
        Cell string `json:"cell"` 
} 






func main() { 

        res2B := &JSONDoc{} 
    fmt.Println("I can create a skeleton json doc") 
    MarshalIt(res2B) 

    fmt.Println("\nI can set value of top level key that is an array.") 
        res2B.JustArray = []string{"One"} 
    MarshalIt(res2B)    

    fmt.Println("\nI can append this top level array.") 
        res2B.JustArray = append(res2B.JustArray, "Two") 
    MarshalIt(res2B) 

    fmt.Println("\nI can set value of a nested key.") 
        res2B.NestedString.Name.First = "Dave" 
        MarshalIt(res2B) 


    fmt.Println("\nHow in the heck do I populate, and append a nested array?") 


} 

func MarshalIt(res2B *JSONDoc){ 
        res, _ := json.Marshal(res2B) 
        fmt.Println(string(res)) 
}

Thanks for any help.

Ainar-G · Accepted Answer · 2016-10-10 16:07:37Z

1

Instead of defining NestedArray as a slice of anonymous structs, it's better to redefine it in JSONDoc as such:

type JSONDoc struct {
    JustArray    []string          `json:"justanarray"`
    NestedString NestedString      `json:"nestedstring"`
    NestedArray  []NestedArrayElem `json:"nestedarray"`
}

//Nested array
type NestedArrayElem struct {
    Address Address `json:"address,omitempty"`
    Phone   Phone   `json:"phone,omitempty"`
}

Then, you can do:

res2B := &JSONDoc{}
res2B.NestedArray = []NestedArrayElem{
    {Address: Address{Street: "foo"}},
    {Phone: Phone{Cell: "bar"}},
}
MarshalIt(res2B)

Playground: https://play.golang.org/p/_euwT-TEWp.

answered Oct 10, 2016 at 16:07

Ainar-G

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3 Comments

sneeze Over a year ago

Thanks Ainar-G, but this appears to have the structure of "nestedarray": [ { "address": {"street": "foo"}, "phone": {"cell": ""} }, { "address": {"street": ""}, "phone": {"cell": "bar"} } ]`

sneeze Over a year ago

Ainar-G, Thanks again, I was able to modify your answer a bit using interfaces, and with your help I believe I am close to a solution. The last piece to my puzzle is this. How do I append? play.golang.org/p/0i0t8O_KrN Thanks!

Ainar-G Over a year ago

@sneeze You don't need interfaces here, pointers are enough. Also, when you append, you should use a value, not a slice. play.golang.org/p/hs4SGHSn7Q

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