<class name="Center" table="Center">
  <id name="Id" column="CenterID" unsaved-value="0">
   <generator class="identity" />
  </id>
  <property name="Name" type="String" length="25" not-null="true" />
  <many-to-one name="Region" column="RegionID" class="Region"
               not-null="true" />
</class>

<class name="Region" table="Region">
  <id name="Id" type="Int32" column="RegionID" unsaved-value="0">
   <generator class="identity" />
  </id>
  <property name="Name" type="String" not-null="true" />
  <set name="Centers" inverse="true" lazy="true" cascade="all">
    <key column="RegionID" />
    <one-to-many class="Center" />
  </set>
</class>

Center c = new Center();
c.Name = "ccc";
Region r = new Region();
r.Name = "rrr";
r.CreditCenters.Add(c);
session.Save(r);

Center c = new Center();
c.Name = "ccc";
Region r = new Region();
r.Name = "rrr";
r.Centers.Add(c);  // instead of r.CreditCenters.Add(c);
session.Save(r);