Remove redundant paren, add test

graphql/id.go

@@ -21,7 +21,7 @@ func UnmarshalID(v interface{}) (string, error) {
 	case int:
 		return strconv.Itoa(v), nil
 	case int64:
-		return strconv.Itoa((int(v))), nil
+		return strconv.Itoa(int(v)), nil
 	case float64:
 		return fmt.Sprintf("%f", v), nil
 	case bool:

graphql/id_test.go

@@ -0,0 +1,35 @@
+package graphql
+
+import (
+	"github.com/stretchr/testify/assert"
+	"testing"
+)
+
+func TestMarshalID(t *testing.T) {
+	tests := []struct {
+		Name string
+		Input interface{}
+		Expected string
+		ShouldError bool
+	}{
+		{
+			Name: "int64",
+			Input: int64(12),
+			Expected: "12",
+			ShouldError: false,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.Name, func(t *testing.T) {
+			id, err := UnmarshalID(tt.Input)
+
+			assert.Equal(t, tt.Expected, id)
+			if tt.ShouldError {
+				assert.Error(t, err)
+			} else {
+				assert.NoError(t, err)
+			}
+		})
+	}
+}