Eager Loading Using Fluent NHibernate/Nhibernate & Automapping

Question

I have a requirement to load a complex object called Node...well its not that complex...it looks like follows:-

A Node has a reference to EntityType which has a one to many with Property which in turn has a one to many with PorpertyListValue

public class Node
{
    public virtual int Id
    {
        get;
        set;
    }

    public virtual string Name
    {
        get;
        set;
    }

    public virtual EntityType Etype
    {
        get;
        set;
    }

}


public class EntityType
{
    public virtual int Id
    {
        get;
        set;
    }

    public virtual string Name
    {
        get;
        set;
    }

    public virtual IList<Property> Properties
    {
        get;
        protected set;
    }

    public EntityType()
    {
        Properties = new List<Property>();
    }
}

public class Property
{
    public virtual int Id
    {
        get;
        set;
    }

    public virtual string Name
    {
        get;
        set;
    }        

    public virtual EntityType EntityType
    {
        get;
        set;
    }

    public virtual IList<PropertyListValue> ListValues
    {
        get;
        protected set;
    }

    public virtual string DefaultValue
    {
        get;
        set;
    }

    public Property()
    {
        ListValues = new List<PropertyListValue>();
    }
}


public class PropertyListValue
{
    public virtual int Id
    {
        get;
        set;
    }

    public virtual Property Property
    {
        get;
        set;
    }

    public virtual string Value
    {
        get;
        set;
    }

    protected PropertyListValue()
    {
    }
}

What I a trying to do is load the Node object with all the child objects all at once. No Lazy load. The reason is I have thousands of Node objects in the database and I have to send them over the wire using WCF Service.I ran into the classes SQL N+ 1 problem. I am using Fluent Nhibernate with Automapping and NHibernate Profiler suggested me to use FetchMode.Eager to load the whole objects at once. I am using the following qyuery

     Session.CreateCriteria(typeof (Node))
            .SetFetchMode( "Etype", FetchMode.Join )
            .SetFetchMode( "Etype.Properties", FetchMode.Join )
            .SetFetchMode( "Etype.Properties.ListValues", FetchMode.Join )

OR using NHibernate LINQ

        Session.Linq<NodeType>()
         .Expand( "Etype")
         .Expand( "Etype.Properties" )
         .Expand( "Etype.Properties.ListValues" )

When I run any of the above query, they both generate one same single query with all the left outer joins, which is what I need. However, for some reason the return IList from the query is not being loaded property into the objects. Infact the returned Nodes count is equal to the number of rows of the query, so the Nodes objects are repeated.Moreover, the properties within each Node are repeated, and so do the Listvalues.

So I would like to know how to modify the above query to return all unique Nodes with the properties and list values within them.

On google I found out about DistinctRootEntityResultTransformer but that only resolve the issue for the Root objects. I am still getting duplicates in the child collections. Every root object in the returned list has some weird Cartesian product mess in the child collections with multiple instances of the same entity. Any idea? Awaiting Nabeel — nabeelfarid
I think I have found the solution but I would like to know if its the correct one. The child collections (EType.Properties, Etype.Properties.ListValues) inside root object (Node) are IList. And i read in the documentation that IList can contain duplicates, so if i change IList to ISet/ ICollection, then the query does not load duplicate instances within the child collections. But this solution requires alot of refactoring. I would like to know if there is a way to achieve the same using IList for child collections? Awaiting, Nabeel — nabeelfarid
I have the same issue (using Fetchmode.Eager). I'm pretty dissapointed in NHibernate for this. I would rather have an error than incorrect data. — UpTheCreek

Tim Hoolihan Tim Hoolihan · Accepted Answer · 2010-06-29T18:27:27

each mapping has to have lazy loading off

in Node Map:

Map(x => x.EntityType).Not.LazyLoad();

in EnityType Map:

Map(x => x.Properties).Not.LazyLoad();

and so on...

Also, see NHibernate Eager loading multi-level child objects for one time eager loading

Added:

Additional info on Sql N+1:

http://nhprof.com/Learn/Alerts/SelectNPlusOne

Eager Loading Using Fluent NHibernate/Nhibernate & Automapping

4 Answers