I have an object which contains a property that is the result of an SQL statement. How do I add the SQL statement to my nHibernate mapping file?

Example Object:

public class Library{
    public int BookCount
    {
        get;
        set;
    }
}

Example Mapping File:

<hibernate-mapping>
    <class name="Library" table="Libraries">
        <property name="BookCount" type="int">  <- This is where I want the SQL query to populate the value. ->
    </class>
</hibernate-mapping>

Example SQL Query:

SELECT COUNT(*) FROM BOOKS WHERE BOOKS.LIBRARY_ID = LIBRARIES.ID