use gocmp for json

main.go

package main

import (
	"encoding/json"
	"fmt"

	"github.com/google/go-cmp/cmp"
	"github.com/google/go-cmp/cmp/cmpopts"
)

// s1 and s2 are equal if you sort the slice called SelectedData and ignore the key called "foo"

func main() {
	s1 := []byte(`{ 
   "SelectedData":[ 
      { 
         "ID":5,
         "bla":"hello5",
         "foo":123
      },
      { 
         "ID":1,
         "bla":"hello1",
         "foo":123
      },
      { 
         "ID":2,
         "bla":"hello2",
         "foo":123
      }
   ]
}`)
	s2 := []byte(`{ 
   "SelectedData":[ 
      { 
         "ID":1,
         "bla":"hello1",
         "foo":123
      },
      { 
         "ID":5,
         "bla":"hello5",
         "foo":123
      },
      { 
         "ID":2,
         "bla":"hello2",
         "foo":222
      }
   ]
}`)

	fmt.Println("cmp.Equal:", cmp.Equal(s1, s2, transformJSON, sortIDs))
}

var transformJSON = cmpopts.AcyclicTransformer("ParseJSON", func(in []byte) (out interface{}) {
	if err := json.Unmarshal(in, &out); err != nil {
		return in
	}
	
	if entities, ok := out.(map[string]interface{}); ok {
		if selectedData, ok2 := entities["SelectedData"].([]interface{}); ok2 {
			for _,v := range selectedData {
				if m, ok3 := v.(map[string]interface{}); ok3 {
					delete(m, "foo")
				}
			}
		}
	}
	
	return out
})

var sortIDs = cmpopts.SortSlices(func(x, y interface{}) bool {
	obj1, _ := x.(map[string]interface{})
	obj2, _ := y.(map[string]interface{})
	id1, _ := obj1["ID"].(float64)
	id2, _ := obj2["ID"].(float64)
	return id1 < id2
})