Post.cs

    public class Post
    {
        public virtual int Id { get; set; }
        public virtual string Title { get; set; }
        public virtual string Body { get; set; }
        public virtual DateTime PubDate { get; set; }
        public virtual ICollection<Tag> Tags { get; set; }

    }

post.hbm.xml

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"
				   assembly="..."
				   namespace="...">
	<class name="Post" table="Posts" >

		<id name="Id">
      <generator class="identity"/>
		</id>
		<property name="Title" not-null="true" length="256"/>
    <property name="Body" not-null="true"/>
    <property name="PubDate" not-null="true"/>
    <set name="Tags" table="Tags" inverse="true" cascade="all-delete-orphan">
      <key column="PostId" />
      <one-to-many class="Tag" />    
    </set>
    </class>
</hibernate-mapping>

Tag.cs

    public class Tag
    {
        public virtual int Id { get; set; }
        public virtual string Name { get; set; }
     
        public virtual Post Parent { get; set;  }

    }

tag.hbm.xml

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"
				   assembly="..."
				   namespace="...">
	<class name="Tag" table="Tags" >

		<id name="Id">
      <generator class="identity"/>
		</id>
    <property name="Name" not-null="true" length="64"/>
    <many-to-one name="Parent" column="PostId" not-null="true"/>
    <!--
    <set name="Posts" table="Posts" >
        <key column="Id"></key>
        <many-to-many class="Post" column="Id"></many-to-many>
    </set>
    -->
    </class>
</hibernate-mapping>