How to map many to many association in NHibernate

I have some database tables named Project, Employee and Branch. An employee can work simultaneously on more than one project. Similarly, in a project, there are multiple employees. Also, a project is conducted at a particular branch. To maintain all these relationships, I am using a project_employee_branch table, which will store the related primary keys of the above three tables. As an example, this project_employee_branch table may contain a row like (1,2,3), which means the project whose primary key is开发者_如何学C 1, is conducted at branch whose primary key is 3, and one of its project member is an employee whose primary key is 2.

How can I map all these associations in NHibernate? I have mapped many-to-one association using foreign key concept, but I don't know how to map these types of associations, where an intermediate table is involved.

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First point I'd make is that your database schema and your description don't match, so please take any advice below in the light of that initial caveat. You say that

a project is conducted at a particular branch

which implies there should be a simple foreign key relationship from project to branch. And of course, if this is what the schema looked like, you would have a two-way many-to-many link table and your life would be much easier.

Anyway, with the three-way combination you have, you need to have a collection of components, where the components have many-to-one properties for the other two object types. There is an example in section 7.2 of the NHibernate documentation, but I think it would look something like this in the mapping for Product:

<set name="BranchEmployees" table="product_employee_branch" lazy="true">
    <key column="product_id">
    <composite-element class="Purchase">
        <many-to-one name="Branch" class="Branch" />
        <many-to-one name="Employee" class="Employee"/>
    </composite-element>
</set>