Merging slices of structs

Question

I'm attempting to merge two different json array's of structs together into one json blob (from separate pagination requests). I'm however, unable to merge them together:

package main

import (
  "encoding/json"
  "log"
)

func main() {
  superset := `[{"id": 1231, "name": "testing1"},{"id": 1235, "name": "testing2"}]`
  subset   := `[{"id": 1237, "name": "testing3"}]`

  s, _ := json.Marshal(superset)
  log.Printf(string(s))

  u, _ := json.Marshal(subset)
  log.Printf(string(u))

   for i := range s {
     u = append(u, s[i])
   }

   log.Printf(string(u))
}

However, that gives me really just the concatenation of them:

"[{\"id\": 1237, \"name\": \"testing3\"}]""[{\"id\": 1231, \"name\": \"testing1\"},{\"id\": 1235, \"name\": \"testing2\"}]"

I'm hoping for an output that looks like:

[{"id": 1237, "name": "testing3"},{"id": 1231, "name": "testing1"},{"id": 1235, "name": "testing2"}]

I feel like I'm missing something silly. Any help would be greatly appreciated. Thank you!

Answer 1

json.Marshal() marshals the given value into JSON (and returns it as a []byte ), so what you have in s and u are JSON texts holding your input JSON texts. Appending (concatenating) these 2 texts will not even result in valid JSON, obviously not what you want.

Your inputs are JSON texts, holding a JSON array. So what you should do is unmarshal your inputs into Go slices (of type []interface{} ), which you can append, then marshal the result back to JSON.

Something like this:

superset := `[{"id": 1231, "name": "testing1"},{"id": 1235, "name": "testing2"}]`
subset := `[{"id": 1237, "name": "testing3"}]`

var s1, s2 []interface{}

if err := json.Unmarshal([]byte(superset), &s1); err != nil {
    panic(err)
}
if err := json.Unmarshal([]byte(subset), &s2); err != nil {
    panic(err)
}

s2 = append(s2, s1...)

result, err := json.Marshal(s2)
if err != nil {
    panic(err)
}
fmt.Println(string(result))

This outputs (try it on the Go Playground ):

[{"id":1237,"name":"testing3"},{"id":1231,"name":"testing1"},{"id":1235,"name":"testing2"}]