many-to-many mapping in NHibernate

Question

I'm looking to create a many to many relationship using NHibernate. I'm not sure how to map these in the XML files. I have not created the classes yet, but they will just be basic POCOs.

Tables

Person
personId
name

Competency
competencyId
title

Person_x_Competency
personId
competencyId

Would I essentially create a List in each POCO for the other class? Then map those somehow using the NHibernate configuration files?

Answer 1

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="Persons" 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.