feat: null string scanner and marshaller

Author: abdivasiyev

.gitignore

@@ -0,0 +1,4 @@
+.idea
+.DS_STORE
+.vscode
+cmd/

string.go

@@ -0,0 +1,75 @@
+package null
+
+import (
+	"database/sql"
+	"database/sql/driver"
+	"encoding/json"
+)
+
+type String struct {
+	nullValue *sql.NullString
+}
+
+func (s String) String() string {
+	if s.nullValue != nil {
+		return s.nullValue.String
+	}
+
+	return ""
+}
+
+func NewString(value string) String {
+	return String{
+		nullValue: &sql.NullString{
+			Valid:  value != "",
+			String: value,
+		},
+	}
+}
+
+func (s String) Get() string {
+	if s.nullValue != nil {
+		return s.nullValue.String
+	}
+
+	return ""
+}
+
+func (s *String) Value() (driver.Value, error) {
+	if s.nullValue != nil {
+		return s.nullValue.Value()
+	}
+
+	return nil, nil
+}
+
+func (s *String) Scan(value any) error {
+	if err := s.nullValue.Scan(value); err != nil {
+		return err
+	}
+
+	return nil
+}
+
+func (s *String) UnmarshalJSON(bytes []byte) error {
+	s.nullValue = &sql.NullString{
+		String: "",
+		Valid:  false,
+	}
+
+	if len(bytes) > 0 {
+		if err := json.Unmarshal(bytes, &s.nullValue.String); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
+func (s String) MarshalJSON() ([]byte, error) {
+	if s.nullValue.Valid && s.nullValue.String != "" {
+		return json.Marshal(s.nullValue.String)
+	}
+
+	return json.Marshal(nil)
+}

string_test.go

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+package null
+
+import (
+	"encoding/json"
+	"reflect"
+	"testing"
+)
+
+func TestString(t *testing.T) {
+	testCases := []struct {
+		name            string
+		s               String
+		marshalledBytes []byte
+	}{
+		{
+			name:            "Not null marshaller",
+			s:               NewString("test"),
+			marshalledBytes: []byte(`"test"`),
+		},
+		{
+			name:            "null marshaller",
+			s:               NewString(""),
+			marshalledBytes: []byte(`null`),
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			bytes, err := json.Marshal(tc.s)
+
+			if err != nil {
+				t.Errorf("could not marshal: %v\n", err)
+				t.FailNow()
+			}
+
+			if !reflect.DeepEqual(bytes, tc.marshalledBytes) {
+				t.Errorf("not equal values => got: %v, want: %v\n", string(bytes), string(tc.marshalledBytes))
+				t.FailNow()
+			}
+		})
+	}
+}