Given a domain model for a simplified todo app

type Todo struct {
    title          string
    isMarkedAsDone bool
    modifiedAt     time.Time
}

func NewTodo(title string) (*Todo, error) {
    if title == "" {
        return nil, errors.New("title is empty")
    }

    todo := &Todo{
        title:          title,
        isMarkedAsDone: false,
        modifiedAt:     time.Now(),
    }

    return todo, nil
}

func (t *Todo) GetTitle() string {
    return t.title
}

func (t *Todo) IsMarkedAsDone() bool {
    return t.isMarkedAsDone
}

// other getters...

func (t *Todo) Rename(newTitle string) error {
    if t.isMarkedAsDone {
        return errors.New("todo is already marked as done")
    }

    if newTitle == "" {
        return errors.New("new title is empty")
    }

    t.title = newTitle
    t.modifiedAt = time.Now()

    return nil
}

func (t *Todo) MarkAsDone() error {
    if t.isMarkedAsDone {
        return errors.New("todo is already marked as done")
    }

    t.isMarkedAsDone = true
    t.modifiedAt = time.Now()

    return nil
}

// other setters...

Saving this todo to a store is no problem since I can access the fields via getters. But when I query the store for todos that are marked as done I can’t reconstruct a domain object from the returned data.

The constructor function takes no isMarkedAsDone = true parameter ( + isMarkedAsDone ) and if I would try to create a new domain todo and call the MarkAsDone function on it I would overwrite the field modifiedAt which is wrong then.

What is a common approach to solve this?

• Make everything public? ( feels wrong to me, consumers could put the domain object in invalid state )
• Change the whole constructor function to accept all fields and validate all of them, so consumers have to provide all fields from outside?
• Keep everything as is but provide a reconstruct function in the same package accepting all fields ( + validation ) that creates a domain model and has write access to the private fields since it lives in the same package?