<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" namespace="Domain" assembly="Domain">
  <class name="BookInfo" table="Books">
    <id name="ID" unsaved-value="0">
      <column name="ID" not-null="true" />
        <generator class="identity"/>
    </id>
    
    <property name="ISBN" />
    <property name="Title" />
    <property name="IdTheme" column="Id_Theme"/>
    <property insert="false" update="false" name="Theme" column="Theme" />
    <loader query-ref="sql"/>
        
  </class>

  <sql-query name="sql">
    select b.*, t.name as Theme from books as b, theme as t where b.id_theme = t.id    
  </sql-query>
</hibernate-mapping>

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" namespace="Domain" assembly="Domain">
  <class name="BookInfo" table="Books">
    <id name="ID" unsaved-value="0">
      <column name="ID" not-null="true" />
        <generator class="identity"/>
    </id>
    
    <property name="ISBN" />
    <property name="Title" />
    <property name="IdTheme" column="Id_Theme"/>
    <property insert="false" update="false" name="Theme" column="Theme" />
    <loader query-ref="loadBooks"/>
    
    
  </class>

  <sql-query name="loadBooks" callable="true">
    <return alias="bi" class="BookInfo" />
    <![CDATA[select bi.id as {bi.ID}, 
        bi.title as {bi.Title}, 
        bi.isbn as {bi.ISBN}, 
        bi.id_theme as {bi.IdTheme}, 
        t.name as {bi.Theme} 
    from books as bi, theme as t 
    where bi.id_theme = t.id and bi.id=:id]]> 
  </sql-query>
</hibernate-mapping>

ICriteria criteria = session.CreateCriteria(typeof(BookInfo));
list = criteria.List<BookInfo>();