Toplink, Many-to-many relationship with date dependency

Hi.
We have an application which manages membership of users in groups.
It is typical many-to-many relationship (table users and groups and
relational table usersgroups). So far so good.
But we would like to introduce another condition into this relationship
- date validity.
For example - person should belong to group only from date1 to date2.

I think that relational table should contain not only a couple of id's but also
a two dates (from-to) and all those four together should form primary key.
Then we should need a condition (where) for such relationsphip.
But toplink many-to-many mapping doesn't have any features for this.

Have you any idea how to implement our requirement in another way?

Thanks.

Generally when your join table has more columns in it you need to map it as an object.

See,
http://en.wikibooks.org/wiki/Java_Persistence/ManyToMany#Mapping_a_Join_Table_with_Additional_Columns

TopLink (EclipseLink) does have support for historization, and a ManyToManyMapping can make use of a HistoryPolicy with a start/end date.
Normally history would be used for the entire object/session, but you may wish to investigate it.

You can also set the selectionCriteria on a mapping to use more advanced criteria than foreign keys.