Question

I have two tables with a many-to-many relation with a bridge table.

User Class Mapping :

<class name="MatrixCore.User" table="MatrixUser" lazy="false">
    <id name="ID" column="ID" unsaved-value="0">
      <generator class="native"/>
    </id>
    <property name="FirstName"/>
    <property name="LastName"/>
    <property name="UserName"/>
    <property name="Password"/>
    <many-to-one name="UserType" class="MatrixCore.UserType" />
    <set name="Projects" table="UserInProject" cascade="All">
      <key column="MatrixUser_ID"/>
      <many-to-many class="Project" column="Project_ID"/>
    </set>
  </class>

Project class mapping :

<class name="MatrixCore.Project" table="Project" lazy="false">
   <id name="ID" column="ID" unsaved-value="0">
     <generator class="native"/>
   </id>
   <property name="Name" />
   <property name="Acronym"/>
   <property name="StartDate"/>
   <property name="EndDate"/>
   <set name="Users" table="UserInProject" cascade="All">
     <key column="Project_ID"/>
     <many-to-many class="User" column="MatrixUser_ID" />
   </set>
</class>

the classes implementations are too simple each class have a collection
of the other.
I am trying to insert records in the tables the bridge table keep to be empty.

    ICollection<Project> ps = new HashSet<Project>() { project};

            UserType tp = (UserType)session.Get("UserType", 1);
            User u = new User()
            {
                FirstName = "Hussein",
                LastName = "Hussein",
                UserName = "Hussein",
                Password = "welcome",
                UserType = tp,
                Projects = ps
            };

            session.Save(u);

Answer 1

try this mapping
for user class

<set name="Projects" table="UserInProject" inverse="true" cascade="save-update" lazy="false">
  <key column="MatrixUser_ID" />
  <many-to-many class="Project" column="Project_ID"/>
</set>

and for Project Class

<set name="Users" table="UserInProject" cascade="save-update" lazy="false">
  <key column="Project_ID"  />
  <many-to-many class="User" column="MatrixUser_ID"/>
</set>