Eager Loading Using Fluent NHibernate/Nhibernate & Automapping

问题

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.

Thanks Nabeel

回答1:

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

回答2:

I figure it out myself. The key is to use SetResultTransformer() passing an object of DistinctRootEntityResultTransformer as a parameter. So the query now looks like as follows

Session.CreateCriteria(typeof (Node))
   .SetFetchMode( "Etype", FetchMode.Join )
   .SetFetchMode( "Etype.Properties", FetchMode.Join )
   .SetFetchMode( "Etype.Properties.ListValues", FetchMode.Join )
   .SetResultTransformer(new DistinctRootEntityResultTransformer());

I found the answer to my questions through these links:

http://www.mailinglistarchive.com/html/nhusers@googlegroups.com/2010-05/msg00512.html

http://ayende.com/Blog/archive/2010/01/16/eagerly-loading-entity-associations-efficiently-with-nhibernate.aspx

回答3:

I ended up with something like this:

HasMany(x => x.YourList).KeyColumn("ColumnName").Inverse().Not.LazyLoad().Fetch.Join()

Just make sure to select your entity like this, to avoid duplication due to the join:

session.CreateCriteria(typeof(T)).SetResultTransformer(Transformers.DistinctRootEntity).List<T>();

回答4:

SetResultTransformer with DistinctRootEntityResultTransformer will only work for Main object but IList collections will be multiplied.

来源：https://stackoverflow.com/questions/3142845/eager-loading-using-fluent-nhibernate-nhibernate-automapping