You can put the many-to-many relation to either class, or even to both. This is up to your domain model. If you map it to both, one of them is inverse.
class Person
{
// id ...
IList<Competency> Competencies { get; private set; }
// you domain model is responsible to manage bidirectional dependencies.
// of course this is not a complete implementation
public void AddCompetency(Competency competency)
{
Competencies.Add(competency);
competency.AddPerson(this);
}
}
class Competency
{
// id ...
IList<Person> Persons { get; private set; }
}
Mapping:
<class name="Person">
<id ....>
<bag name="Competencies" table="Person_x_Competency">
<key column="personId"/>
<many-to-many class="Competency" column="competencyId"/>
</bag>
</class>
<class name="Competency">
<id ....>
<bag name="Competencies" table="Person_x_Competency" inverse="true">
<key column="competencyId"/>
<many-to-many class="Person" column="personId"/>
</bag>
</class>
Only make it bidirectional if you really need it.
By the way: it is much better to write the classes first and create the database design afterwards. The database can be exported from the mapping files. This is very useful.