Say I have a Department object, which has a "map", contains multiple Users.

The <index> will be each User's "ID" column, which will be used as "key" for the IDictionary type.

The <key> will be each User's "DepartmentID" column.

If I remove a User from the Department, I just remove the user object instance from the IDictionary. When I use session.save() to save the owner object -- Department, it cleans the "DepartmentID"' column of that user (which is expected), it also cleans the "ID" column (which it should not be clean, because the user's ID shall be kept forever, and actually, it's not null column.

If I use <many-to-many>, the remove will delete the whole record. If I use <Composite-element> the remove doesn't do anything[/code]

It seems (N)Hibernate enforce a seperate column for <map>'s <index>.

That <index> is the "Key" of IDictionary, usually, it is the Value's identity or name property.

Say, Teacher vs. Students, many-to-many relationship. I'd like to have a Hashtable in Teacher class, which contains all the Students belont to one teacher. The Hashtable's key is the student's name or ID, the value the the object instance of the student, sure, the student object still have name and ID property, which get from the DB.

Why (N)Hibernate enforce <index> has to be a seperate column?