Handle type mismatch

appstore/model.go

@@ -1,5 +1,18 @@
 package appstore
 
+import "encoding/json"
+
+type numericString string
+
+func (n *numericString) UnmarshalJSON(b []byte) error {
+	var number json.Number
+	if err := json.Unmarshal(b, &number); err != nil {
+		return err
+	}
+	*n = numericString(number.String())
+	return nil
+}
+
 type (
 	// https://developer.apple.com/library/content/releasenotes/General/ValidateAppStoreReceipt/Chapters/ValidateRemotely.html
 	// The IAPRequest type has the request parameter
@@ -76,11 +89,11 @@ type (
 	Receipt struct {
 		ReceiptType                string        `json:"receipt_type"`
 		AdamID                     int64         `json:"adam_id"`
-		AppItemID                  int64   `json:"app_item_id"`
+		AppItemID                  numericString `json:"app_item_id"`
 		BundleID                   string        `json:"bundle_id"`
 		ApplicationVersion         string        `json:"application_version"`
 		DownloadID                 int64         `json:"download_id"`
-		VersionExternalIdentifier  int64   `json:"version_external_identifier"`
+		VersionExternalIdentifier  numericString `json:"version_external_identifier"`
 		OriginalApplicationVersion string        `json:"original_application_version"`
 		InApp                      []InApp       `json:"in_app"`
 		ReceiptCreationDate

appstore/model_test.go

@@ -0,0 +1,58 @@
+package appstore
+
+import (
+	"encoding/json"
+	"errors"
+	"reflect"
+	"testing"
+)
+
+func TestNumericString_UnmarshalJSON(t *testing.T) {
+	type foo struct {
+		ID numericString
+	}
+
+	tests := []struct {
+		name string
+		in   []byte
+		err  error
+		out  foo
+	}{
+		{
+			name: "string case",
+			in:   []byte("{\"ID\":\"8080\"}"),
+			err:  nil,
+			out:  foo{ID: "8080"},
+		},
+		{
+			name: "number case",
+			in:   []byte("{\"ID\":8080}"),
+			err:  nil,
+			out:  foo{ID: "8080"},
+		},
+		{
+			name: "object case",
+			in:   []byte("{\"ID\":{\"Num\": 8080}}"),
+			err:  errors.New("json: cannot unmarshal object into Go struct field foo.ID of type json.Number"),
+			out:  foo{},
+		},
+	}
+
+	for _, v := range tests {
+		t.Run(v.name, func(t *testing.T) {
+			out := foo{}
+			err := json.Unmarshal(v.in, &out)
+
+			if err != nil {
+				if err.Error() != v.err.Error() {
+					t.Errorf("input: %s, get: %s, want: %s\n", v.in, err, v.err)
+				}
+				return
+			}
+
+			if !reflect.DeepEqual(out, v.out) {
+				t.Errorf("input: %s, get: %v, want: %v\n", v.in, out, v.out)
+			}
+		})
+	}
+}