Nhibernate: How to represent Many-To-Many relationships with One-to-Many relationships?

I just come up to that question, and realized, that there is missing any answer. And it is a shame, while I do often point out this NHibernate documentation statement: 24. Best Practices

Don’t use exotic association mappings.

Good usecases for a real many-to-many associations are rare. Most of
the time you need additional information stored in the “link table”.
In this case, it is much better to use two one-to-many associations to
an intermediate link class. In fact, we think that most associations
are one-to-many and many-to-one, you should be careful when using any
other association style and ask yourself if it is really neccessary.

Take a look at the example under the 23.2. Author/Work. Extract, the simplified version of the many-to-many relation between Author and Work:

<class name="Work" table="works" ...>
        <id name="Id" column="id" generator="native" />
        ...
        <set name="Authors" table="author_work" lazy="true">
            <key>
                <column name="work_id" not-null="true"/>
            </key>
            <many-to-many class="Author">
                <column name="author_id" not-null="true"/>
            </many-to-many>
        </set>
</class>

And its many-to-many target Author:

<class name="Author" table="authors">
  ...
  <set name="Works" table="author_work" inverse="true" lazy="true">
     <key column="author_id"/>
     <many-to-many class="Work" column="work_id"/>
  </set>
</class>

So, if we would like to order the set of Works on load, we do have a problem. There is no column in the pair table. But what’s more important, there is no way how to manage such a column.

What we can do, is to introduced the Pair object: AuthorWork and extend the Pair table as needed

public class AuthorWork
{

    public virtual Author Author { get; set; }
    public virtual Work Work { get; set; }
    public virtual int OrderBy { get; set; }
}

Mapping of the AuthorWork

<class name="AuthorWork" table="author_work">
    ...
    <many-to-one name="Author" column="author_id" />
    <many-to-one name="Workr"  column="work_id" />
    <property name="OrderBy" />

Having this we can convert the many-to-many mapping to one-to-many, for example the Authors collection:

<set name="Authors" lazy="true"
  order-by="OrderBy">
  <key column="work_id" not-null="true"/>
  <one-to-many class="AuthorWork" />
</set>

And we can manage the entity AuthorWork, set the OrderBy column, and therefore effectively work with the pairing table.

NOTE: have to agree with that suggestion in docsumentation The more requirements come, the more happy we are that we do have a way how to manage the relation!