Add JSON-encoded version of NULL uniqueidentifier (#238)

* Add JSON-encoded version of NULL uniqueidentifier

* as literal name according RFC 7159

* Fix test to verify that Marshal writes the correct value

Author: sedbygo

uniqueidentifier_null.go

@@ -2,6 +2,7 @@ package mssql
 
 import (
 	"database/sql/driver"
+	"encoding/json"
 )
 
 type NullUniqueIdentifier struct {
@@ -63,3 +64,10 @@ func (n *NullUniqueIdentifier) UnmarshalJSON(b []byte) error {
 	}
 	return err
 }
+
+func (n NullUniqueIdentifier) MarshalJSON() ([]byte, error) {
+	if !n.Valid {
+		return []byte("null"), nil
+	}
+	return json.Marshal(n.UUID)
+}

uniqueidentifier_null_test.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"database/sql"
 	"database/sql/driver"
+	"encoding/json"
 	"fmt"
 	"reflect"
 	"testing"
@@ -210,6 +211,42 @@ func TestNullableUniqueIdentifierUnmarshalJSONNull(t *testing.T) {
 	}
 }
 
-var _ fmt.Stringer = NullUniqueIdentifier{}
-var _ sql.Scanner = &NullUniqueIdentifier{}
-var _ driver.Valuer = NullUniqueIdentifier{}
+func TestNullableUniqueIdentifierMarshalJSONNull(t *testing.T) {
+	t.Parallel()
+	nullUUID := NullUniqueIdentifier{
+		UUID:  [16]byte{0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0},
+		Valid: false,
+	}
+
+	got, err := nullUUID.MarshalJSON()
+	if err != nil {
+		t.Fatal(err)
+	}
+	want := []byte{0x6e, 0x75, 0x6c, 0x6c} // null = %x6e.75.6c.6c
+	if !reflect.DeepEqual(got, want) {
+		t.Errorf("got %v; want %v", got, want)
+	}
+}
+
+func TestNullableUniqueIdentifierJSONMarshalNull(t *testing.T) {
+	t.Parallel()
+	nullUUID := NullUniqueIdentifier{
+		UUID:  [16]byte{0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0},
+		Valid: false,
+	}
+
+	got, err := json.Marshal(nullUUID)
+	if err != nil {
+		t.Fatal(err)
+	}
+	want := []byte{0x6e, 0x75, 0x6c, 0x6c}
+	if !reflect.DeepEqual(got, want) {
+		t.Errorf("got %v; want %v", got, want)
+	}
+}
+
+var (
+	_ fmt.Stringer  = NullUniqueIdentifier{}
+	_ sql.Scanner   = &NullUniqueIdentifier{}
+	_ driver.Valuer = NullUniqueIdentifier{}
+)