Tags mapping in NHiberbate

When I'm asking this question on stackoverflow, I can add tags to it. So, in DDD, I would have a class Question that somehow has tags. One way to model it would be with a List of tags, since a tag is not really an entity (or is it?).

public class Question
{
    // ...

    public List<string> Tags;
}

In the database, however, I would probably have these three tables:

Question: QuestionID, Title, Content
Tag: TagID, Title
QuestionTag: QuestionID, TagID

Would this be 开发者_开发技巧possible from an NHibernate perspective? Would it be recommended? How would you do it?

It depends on:

• how you are creating / changing / deleting tags
• how you query for tags and questions by tags
• how many items you have and if you desperately need performance tricks

You could:

• map the tags as an entity, with an id, which is reused. There will be a separate class (Tag) and two tables, a Tag table and a QuestionToTag table (and additional XxxToTag table for each Tag owner if there are others).
• map the tags as a list of strings, which are not reused. You need no special class and thee is one additional Table for each Tag owner (QuestionTags).
• map the tags as a single string with separated by some character, eg. a comma or a space. There is no class and no table for this.

You can't have tags in a list of strings which are reused for several questions. If they should be reused, they need to be entities.

Every solution I proposed has its advantages and disadvantages. It depends on what is most important for you to say which solution is the best.

While what you describe is possible you may want to consider Tag as an entity. Consider the situation where you want to display a list of questions for a particular Tag. It will therefore result in a many to many relationship between the Tag entity and the Question entity. The database structure will pretty much end up the way you envision.