Effectively retrieving object with nested collections using NHibernate - nhibernate

Question

I am creating a survey application where I have a survey which has a collection of pages. Each page will have a collection of questions and each question will have a collection of answer options. My class structure looks like:
public class Survey : Entity {
public IList<Page> Pages { get; set; }
}
public class Page : Entity {
public IList<Question> Questions { get;set; }
}
public class Question : Entity {
public IList<Option> Options { get; set; }
}
public class Option : Entity {}
The mapping for each class is:
<!-- mapping for ID and other properties excluded -->
<class name="Survey">
<bag name="Pages" generic="true" inverse="true">
<key column="SurveyId" />
<one-to-many class="Page" />
</bag>
<bag name="Questions" access="none">
<key column="SurveyId" />
<one-to-many class="Question" />
</bag>
</class>
<class name="Page">
<many-to-one name="Survey" column="SurveyId" />
<bag name="Questions" generic="true" inverse="true">
<key column="PageId" />
<one-to-many class="Question" />
</bag>
</class>
<class name="Question">
<many-to-one name="Page" column="PageId" />
<many-to-one name="Survey" column="SurveyId" />
<bag name="Options" generic="true" inverse="true">
<key column="QuestionId" />
<one-to-many class="Option" />
</bag>
</class>
<class name="AnswerOption">
<many-to-one name="Question" column="QuestionId" />
</class>
I need to display all the questions on a page so I start with the survey object and loop through the pages, items and options. This causes NHibernate to execute many queries and I would like to optimize this. How can I get the survey object with the nested collections in the best possible way without executing too many queries?
This is the code I have at the moment but it still executes many queries:
var result = Session.CreateMultiQuery()
.Add(Session.CreateQuery("from Survey s inner join fetch s.Pages where s.Id = :id"))
.Add(Session.CreateQuery("from Survey s inner join fetch s.Question where s.Id = :id"))
.SetInt32("id", id)
.List();
IList list = (IList)result[0];
return list[0] as Survey;
I have also tried Future queries but they don't help to reduce the number of queries.
Any ideas?

Answer 1

If I understood correctly, you can achieve this with the following HQL (of course the same idea can be used with ICriteria)
from Survey s
inner join fetch s.Pages p
inner join fetch p.Questions q
inner join fetch q.Options
where s.Id = :id
If you also want to fetch survey.Questions then the best option is to fetch those in separate qyery and used Futures to avoid cartesian product.
Also, if I remember correctly HQL queries ignores fetch's defined in mappings. If the collection is mapped with lazy="false" fetch="join" then it is fetch when used ICriteria but not when using HQL.

Answer 2

You could try adding
lazy="false" fetch="join"
to your bag declarations. That way you can be sure the bag will be fetched using one query.

Answer 3

A solution I'm using is reversing relation between entities. With this you can be sure when a record of Survey is loaded, no record of Page is loaded unless you call LoadAll method. LoadAll is a method in each class that search all related records. Consider following code:
public class Survey : Entity {
Survey[] LoadAll {
string hql = "from Page page where page.SurveyID =" + this.ID;
//....
}
}
public class Page : Entity {
public Survey { get;set; }
}
public class Question : Entity {
public Page Page { get; set; }
}
public class Option : Entity {
public Question Question {set; get;}
}