R – Best way to implement functionality similar to lazy=“extra” using NHibernate 2.0.1

nhibernate

I have a many-to-one relationship where the child table can have hundreds of thousands of records. In this case, calling Parent.ChildCollection.Count forces a lazy initialization of the child collection which is extremely expensive.

In Hibernate 3.0 there is a feature lazy="extra" which allows you to check a subset of collection properties without lazy loading the whole thing.

Unfortunately this will not be available until NHibernate 2.1, which is still in Alpha.
http://jira.nhibernate.org/browse/NH-855

How can I accomplish this with NHibernate 2.0.1?

I used to have special properties such as this

<property name="ChildCollectionCount" type="int" formula="(select count(*) from ChildTable child where child.parentID = parentID "/>

but I can't use these anymore because I am now sharing this library and its a performance problem for other users.

Best Answer

When you say its a performance problem for other users, do you mean they also want to access the collection, but its too large for them. Or is the code snippet/ChildCollectionCount too slow for them?

If its the first case, then perhaps you need to do a similar solution for them - identify exactly what they need and provide methods that offer that facility.

Related Solutions

C# – Best way to manage session in NHibernate

Session management:

http://code.google.com/p/dot-net-reference-app/source/browse/trunk/src/Infrastructure/Impl/HybridSessionBuilder.cs

Session per request:

http://code.google.com/p/dot-net-reference-app/source/browse/trunk/src/Infrastructure/Impl/NHibernateSessionModule.cs

Nhibernate – disable automatic\lazy loading of child records for one to many relationsihps

Given your request, you could simply not map from Department to Employees, nor have an Employees property on your department. This would mean you always have to make a database hit to find the employees of a database.

Aplogies if these code examples don't work out of the box, I'm not near a compiler at the moment

So, your department class might look like:

 public class Department 
 { 
     public int Id { get; protected set; }
     public string Name { get; set; }
     /* Equality and GetHashCode here */
 }

and your Employee would look like:

 public class Employee
 { 
     public int Id { get; protected set; }
     public Name Name { get; set; }
     public Department Department { get; set; }
     /* Equality and GetHashCode here */
 }

Any time you wanted to find Employees for a department, you've have to call:

/*...*/
session.CreateCriteria(typeof(Employee))
    .Add(Restrictions.Eq("Department", department)
    .List<Employee>();

Simply because your spec says "Departments have many Employees", doesn't mean you have to map it as a bi-directional association. If you can keep your associated uni-directional, you can really get your data-access to fly too.

Google "Domain Driven Design" Aggregate, or see Page 125 of Eric Evan's book on Domain Driven Design for more information

Best Answer

Related Solutions

C# – Best way to manage session in NHibernate

Nhibernate – disable automatic\lazy loading of child records for one to many relationsihps

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