How to handle two entities with one “unique” property?

Question

I have these entities:

public class User {
    public string Username { get; set; }
    // OtherProperties
}

public class Page {
    public string Username { get; set; }
    // OtherProperties
}

OtherProperties are completely different in each entity from another one. But, Usernames should be unique, no matter where they belong. I mean if there is a User with Username = a, none of other Users or Pages can have this Username. How would you handle this? Is it a good idea to have a 3th entity e.g. a

class Username { string Name { get; set; } }

considering this fact that it is just a simple-single property?

UPDATE: more explain:

User is a completely independent different entity from Page. But both of them have a property named Username which should be unique in entire application.

@NikhilAgrawal Would you post your comment as an answer? So I can +1 it or probably accept that. — amiry jd, Jul 14 '14 at 06:29

score 1 · Accepted Answer · answered Jul 14 '14 at 06:51

Yes, your approach is good. Create a third class Username and add a property of it in your first two classes. Like this

class UserName
{
    string Name { get; set; } 
}

public class User
{
    public Username UName { get; set; }
    // OtherProperties
}

public class Page 
{
    public UserName UName { get; set; }
    // OtherProperties
}

Changed property name because class Name and property name was same. You can have what suits you best.

Back to problem. Moreover tomorrow if you want to add any other details about User Name, then you have to add a property in your UserName and your code in User and Page will not require any change.

score 0 · Answer 2 · edited May 23 '17 at 12:11

If you want to have unique user names in many classes then you should implement 3rd table with unique usernames which will be FKyed to all of the tables containing username property (column). For this task you can use class attributes or FluentAPI. As my experience told me it is better to use FluentAPI as attributes approach not always provide expected results.

For example smth like this:

public class Username
{
    [Index(IsUnique = true)]
    public int Id {get;set;}
    [Index(IsUnique = true)]
    public string Name {get;set;}

    public virtual ICollection<Page> PagesList{ get; set; }
}

public class Page {
    public int UsernameId { get; set; }
    // OtherProperties
    public virtual string Username {get;set;}
}

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
        //one-to-many 
        modelBuilder.Entity<Page>().HasRequired<Username>(s => s.Username)
        .WithMany(s => s.PagesList)
        .WillCascadeOnDelete(true);
}

More details on FluentAPI unique keys: Unique keys in Entity Framework 4

More details on implementing FKs: http://www.entityframeworktutorial.net/code-first/configure-one-to-many-relationship-in-code-first.aspx

No this is not what I mean. `User` is a completely different entity from `Page`. But both of them have a property named `Username` which should be unique in entire application. — amiry jd, Jul 14 '14 at 06:21
Then you shoud make 3rd table that is common for both of them. — Alexander Smirnov, Jul 14 '14 at 06:24
OK. I updated the question too. If you think I should do that, post it as your answer - or update the current one - so I can +1 it. I'll wait for a while for other suggestions, and I will accept the best one. — amiry jd, Jul 14 '14 at 06:27

How to handle two entities with one “unique” property?

2 Answers2