@GertArnold I think the answer of my previous question lies in your main answer, the look-up table is the way to go for it ;)

@t3chb0t Thanks for your answer, I learned again something new from it! :) As Dmitry Nogin advised, I am going to refactor my code with introducing an abstract container class, so probably the exceptions will not be thrown anymore.

@DmitryNogin Yes, you are right about that I might not need DDD for this application, it is not that complex. It is a real application though, already deployed to production. But I needed a "learning project" in order to learn and apply DDD, and this project is ideal for me, since it is partly mine (used in family business)

@DmitryNogin nice one, I like your design as well! and also thanks for the book recommendations, I am definitely going to read the mentioned parts! ;)

@GertArnold There is of course a workaround for for the mentioned issue, f.e.: nodogmablog.bryanhogan.net/2014/11/… . But I do not want to place data layer relevant logic into my domain layer, it does not belong to it as far as i can judge the situation. Or maybe do you know a better solution for that? :)

@GertArnold Waoo, thank you very much for the detailed answers! I love it! As for your question about using strings instead of enums: I did not like to way that the enum values stored in dabatase as integers. When I read the tables, it was always a pain to figure what is the corresponding enum value of an integer and took time. Check this answer: codereview.stackexchange.com/questions/154676/… .