Question

I have been trying for quite a while to work out how to accomplish my mission, and in fact I am not even sure that the direction I am heading in, is indeed a diserable one in terms of my SQL tables – but it is the most concise I have been able to think up on.

My problem is fairly simple to describe. I have a table containing comments, with a foreign-key to my User table (which represents the author of the comment). In an attempt to properly support anonymous users I have decided to make this foreign key nullable – and I also provide 3 columns for specifying Name, Email and Website where I wish to store the information relevant in case the user is anonymous – these fields are also nullable.

So in short

• If the user is logged in, the foreign-key will contain the ID of that user and Name, Email and Website will be null.
• If the user is not logged in, the foreign-key will be null and Name, Email and Website will contain data about the user.

In my domain model I wish to map this to a User class (a property, Author, on my comment class). This User class then have a property called “IsAnonymous” – which should be set accordingly.

Ideally, I would have some mapping set up that would map this Author property depending on the values of the foreign-key and assign these properties correct. I already have a mapping for my User class that maps it to the User table, but I am not sure how I would do this the other way.

I have tried looking into the IUserType interface, but I can’t quite understand how I would ago about this issue of having to read from 2 different tables.

Is there a reasonable way of doing this using my current database structure – or should I consider switching my layout, and if so, to what?

I switched from having a simple column in my User table indicating an anonymous user, but this started giving me issues when the Name column is set as a unique key. I want registered Users to have unique Name’s but not anonymous Users.

Thanks for your time.

EDIT: Read over that and decided maybe some ASCII/pseudo-SQL diagrams would come in handy

User
-----
UserId Guid PRIMARY KEY
Name nvarchar(200) UNIQUE KEY NOT NULL
Email nvarchar(200) NOT NULL
Website nvarchar(200) NOT NULL

Comment
-------
CommentId Guid PRIMARY KEY
UserId GUID - FK-USER
Name nvarchar(200)
Email nvarchar(200)
Website nvarchar(200)

Answer 1

I’d have something like that

//map this in NH
public virtual User LoggedInCreator {get;set;}
//Not mapped
public virtual User CreatorInformation {
    get {
        if(LoggedInCreator != null) return LoggedInCreator;
        return new User {
                Name = AnonymousCommenterName,
                Email = AnonymousCommenterEmail,
                Website= AnonymousCommenterWebsite
            };
    }
}
public void SetAnonymouscommenter(string name, string email, string website)
{
    LoggedInCreator = null;
    AnonymousCommenterName = name;
    AnonymousCommenterEmail = email;
    AnonymousCommenterWebsite = website;
}

However, if you want to do it in a much cleaner way, you’d use a great feature of NHibernate, which is being able to map Fields and also mapping properties that store their values in fields.
Here is what a cleaner implementation would be:

protected User _commenter;
public virtual User Commenter
{
    get {
        if(_commenter != null) return _commenter;
        return new User {
                Name = AnonymousCommenterName,
                Email = AnonymousCommenterEmail,
                Website= AnonymousCommenterWebsite
        };
    }
    set {
        bool isAnonymous = value.Id == 0;
        _commenter = isAnonymous ? null : value;
        AnonymousCommenterName = isAnonymous ? value.Name : null;
        AnonymousCommenterEmail = isAnonymous ? value.Email : null;
        AnonymousCommenterWebsite = isAnonymous ? value.Website : null;
    }
}

You can see they are essentially the same idea though, just some cleanup using NHibernate features.