<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" assembly="Business" namespace="Business.Entities">
   <class name="Job" table="Job">
      
      <id name="Id" column="JobId" type="Int32" unsaved-value="0">
         <generator class="identity"/>
      </id>
      <property column="InternalId" type="string" name="Title" not-null="true" />
      <property column="Description" type="String" name="Description" not-null="true" />
      <property column="Url" type="String" name="Url" not-null="true" />

    <bag name="CategoryIds" table="JobCategory" fetch="subselect" inverse="true" lazy="true" generic="true" >
      <key column="JobId" />
      <element column="CategoryId" type="Int32"/>
    </bag>

    <bag name="RegionIds" table="JobRegion" fetch="subselect" inverse="true" lazy="true" generic="true" >
      <key column="JobId" />
      <element column="RegionId" type="Int32"/>
    </bag>

   </class>
</hibernate-mapping>

IQuery query = Session.CreateQuery("from Job j where j.Id in (:id_list)");
query.SetParameterList("id_list", jobIds);
IList<Job> result = query.List<Job>();

NHibernateUtil.Initialize(result[0].CategoryIds);
NHibernateUtil.Initialize(result[0].RegionIds);

select job0_.JobId as JobId0_, job0_.Title as Title0_, from Job job0_ where (job0_.JobId in(@p0 , @p1 , ... , @p999))

SELECT categoryid0_.JobId as JobId__0_, categoryid0_.CategoryId as CategoryId0_ FROM JobCategory categoryid0_ WHERE categoryid0_.JobId in (select job0_.JobId from Job job0_ where (job0_.JobId in(@p0 , @p1 , ..., @p999)));

SELECT regionids0_.JobId as JobId__0_, regionids0_.RegionId as RegionId0_ FROM JobRegion regionids0_ WHERE regionids0_.JobId in (select job0_.JobId from Job job0_ where (job0_.JobId in(@p0 , @p1 , @p2 , ..., @p999)));

static void Main(string[] args)
{
   JobService js = new JobService();

   IList<int> ids = js.GetActiveJobIds();
   IList<Job> jobs = js.Get(new List<int>(ids).GetRange(0, 1000), true);
}

  public class JobService
  {
    public IList<Job> Get(List<int> jobIds, bool forceLoadChildren)
    {
      using (PersistenceManager pm = new PersistenceManager())
      {
        IQuery query = pm.Session.CreateQuery("from Job j where j.Id in (:id_list)");

        query.SetParameterList("id_list", jobIds);

        IList<Job> result = query.List<Job>();

        if (forceLoadChildren && (result.Count > 0))
        {
          NHibernateUtil.Initialize(result[0].CategoryIds);
          NHibernateUtil.Initialize(result[0].RegionIds);
        }

        pm.Flush();

        return result;
      }
    }

    public IList<int> GetActiveJobIds()
    {
      using (PersistenceManager pm = new PersistenceManager())
      {
        IQuery query = pm.Session.CreateQuery("select j.Id from Job j where j.Status=:Status");

        query.SetEnum("Status", JobStatuses.Active);

        IList<int> result = query.List<int>();
        pm.Flush();
        return result;
      }
    }
  }

fetch="select"

batch-size="X"

NHibernateUtil.Initialize(result[i].CategoryIds);