NHibernate - Retrieve default data for non-configured fields

Question

I have the following requirement:

User Table

• UserID int (PK)
• UserName string

Configuration Table

• ConfigID int (PK)
• ConfigName string
• DefaultConfigValue string

UserConfig

• UserID int (PK)
• ConfigID int (PK)
• ConfigValue string

How can I define my NHibernate mapping to load all of the configuration with the user value if configured?

Eg.

ConfigID  | DefaultValue 
----------+-------------
1         | 100 
2         | Test


UserID  | ConfigID   |Value
--------+------------+-----
1       | 1          |200

If User 1 logs in then my config table should have the value:

ConfigId | Value
---------+------
1        | 200
2        | Test