NHibernate Mapping a Table Were the Primary Key is Also a Foreign Key - sql

Question

I have 2 tables as follows:
create table Users
(
UserId int primary key identity not null
)
create table UserExternalKeys
(
UserIdRef int primary key not null,
ExternalKey varchar(50) unique not null
)
alter table UserExternalKeys
add constraint fk_UsersExternalKeys_Users
foreign key (UserIdRef)
references Users (UserId)
Each user can have a 0 or 1 external keys. Things are setup this way because adding a nullable unique column to SQL Server does not allow for more than 1 null value.
Based on Ayende's post, it seems like this could be handled using a <one-to-one> mapping. However, this would require the UserExternalKeys table to have its own primary key.
The new schema would look something like this:
create table Users
(
UserId int primary key identity not null,
ExternalKeyRef int null
)
create table UserExternalKeys
(
UserExternalKeyId int primary key identity not null,
ExternalKey varchar(50) unique not null
)
alter table Users
add constraint fk_Users_UsersExternalKeys
foreign key (ExternalKeyRef)
references UserExternalKeys (UserExternalKeyId)
I think this would work, but it feels like I would only be adding the UserExternalKeyId column to appease NHibernate.
Any suggestions?

Answer 1

If a user can have 0 or 1 external keys why not design the tables as:
create table Users
(
UserId int primary key identity not null
ExternalKey varchar(50) null
)
and use one of the known workarounds for this problem. If you're using SQL Server 2008 you can use a filtered index. If you're using an earlier version you can use a trigger, an indexed view (2005), or the nullbuster workaround.
You could also keep your original schema and map the relationship as one-to-many from Users to UserExternalKeys. Map the collection as a private member and expose access to it through a property:
private IList<UserExternalKeys> _externalKeys;
public string ExternalKeys
{
get
{
if (_externalKeys.Count() == 1)
{
return _externalKeys.ElementAt(0).ExternalKey;
}
else
{
// return null or empty string if count = 0, throw exception if > 1
}
}
set
{
if (_externalKeys.Count() == 0) { // add key and set value }
else { // set value if count = 1, throw exception if > 1 }
}
}