Prevent lazy loading in nHibernate

I'm storing some blobs in my database, so I have a Document table and a DocumentContent table. Document contains a filename, description etc and has a DocumentContent property.

I have a Silverlight client, so I don't want to load up and send the DocumentContent to the client un开发者_开发技巧less I explicity ask for it, but I'm having trouble doing this.

I've read the blog post by Davy Brion. I have tried placing lazy=false in my config and removing the virtual access modifier but have had no luck with it as yet.

Every time I do a Session.Get(id), the DocumentContent is retrieved via an outer join. I only want this property to be populated when I explicity join onto this table and ask for it.

Any help is appreciated.

My NHibernate mapping is as follows:

<?xml version="1.0" encoding="utf-8" ?>

<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"
                   assembly="Jrm.Model"
                   namespace="Jrm.Model">
  <class name="JrmDocument" lazy="false">
    <id name="JrmDocumentID">
      <generator class="native" />
    </id>

    <property name="FileName"/>
    <property name="Description"/>
    <many-to-one name="DocumentContent" class="JrmDocumentContent" unique="true" column="JrmDocumentContentID" lazy="false"/>
  </class>


  <class name="JrmDocumentContent" lazy="false">
    <id name="JrmDocumentContentID">
      <generator class="native" />
    </id>

    <property name="Content" type="BinaryBlob" lazy="false">
      <column name="FileBytes" sql-type="varbinary(max)"/>
    </property>

  </class>
</hibernate-mapping>

and my classes are:

[DataContract]   
    public class JrmDocument : ModelBase
    {
        private int jrmDocumentID;
        private JrmDocumentContent documentContent;
        private long maxFileSize;
        private string fileName;
        private string description;

        public JrmDocument()
        {
        }

        public JrmDocument(string fileName, long maxFileSize)
        {
            DocumentContent = new JrmDocumentContent(File.ReadAllBytes(fileName));
            FileName = new FileInfo(fileName).Name;
        }

        [DataMember]
        public virtual int JrmDocumentID
        {
            get { return jrmDocumentID; }
            set
            {
                jrmDocumentID = value;
                OnPropertyChanged("JrmDocumentID");
            }
        }               

        [DataMember]
        public JrmDocumentContent DocumentContent
        {
            get { return documentContent; }
            set
            {
                documentContent = value;
                OnPropertyChanged("DocumentContent");
            }
        }        

        [DataMember]
        public virtual long MaxFileSize
        {
            get { return maxFileSize; }
            set
            {
                maxFileSize = value;
                OnPropertyChanged("MaxFileSize");
            }
        }


        [DataMember]
        public virtual string FileName
        {
            get { return fileName; }
            set
            {
                fileName = value;
                OnPropertyChanged("FileName");
            }
        }

        [DataMember]
        public virtual string Description
        {
            get { return description; }
            set
            {
                description = value;
                OnPropertyChanged("Description");
            }
        }
    }



    [DataContract]
    public class JrmDocumentContent : ModelBase
    {
        private int jrmDocumentContentID;
        private byte[] content;

        public JrmDocumentContent()
        {
        }
        public JrmDocumentContent(byte[] bytes)
        {
            Content = bytes;
        }

        [DataMember]
        public int JrmDocumentContentID
        {
            get { return jrmDocumentContentID; }
            set
            {
                jrmDocumentContentID = value;
                OnPropertyChanged("JrmDocumentContentID");
            }
        }

        [DataMember]
        public byte[] Content
        {
            get { return content; }
            set
            {
                content = value;
                OnPropertyChanged("Content");
            }
        }
    }

---

If you want to defer loading, then set lazy="true" in your map.

---

You should use DTOs for your service and leave all with lazy = true instead of serializing you domain model. It's a big performance gain.

---

First rule of thumb:

• do not distribute your domain objects

http://martinfowler.com/bliki/FirstLaw.html

---

The scenario you have described is exactly what lazy loading is designed for.

You want to display a list containing summary information and when required load the heavier stuff. This is lazy loading. It is "lazy" because it avoids doing extra work until it absolutely needs to.

What you want is for the JrmDocumentContent to be loaded lazily and you're well on th way to that. To get this you have to remove the lazy=false. lazy=true is the default in nhibernate but you may put lazy=true to be sure. You'll have to restore the virtuals though.

Loading blobs lazily like this or any property for that matter, I think is supported in nhibernate's latest release now by setting lazy=true on the property definition.

The method you have used by separating the heavy content into a separate class/table was the only way to lazily load a blob before. I don't know what version of nhibernate you're using but the strategy you have adopted is correct. You should embrace lazy loading. You also need to remove lazy=false on the Content property of the JrmDocumentContent class. Apart from that, I see no other reason why it shouln't work.

---

I've had exactly this problem.

Here's what I've done:

1. Create my own custom proxy (using Castle DynamicProxy, but you can try others) that returns null on all un-initialized properties and collections, and returns a copy of each initialized collection. I wrap each root object result in my service with such a proxy. Pass this proxy as the return value of my service - almost works, except that I couldn't get my proxies themselves to be serialized correctly. That's why I tried method 2:

2. (Horrible) Deeply clone all my entities into new objects, skipping uninitialized properties and collections. This works, but sucks.

3. Currently I'm trying to use DataContractSurrogate to solve the problem in 1. or to replace the use of proxies altogether. The problem I'm currently trying to solve is that you can't "filter" object properties - you can't return null from GetObjectToSerialize - which is exactly what I want to return if the object being serialized is an uninitialized NHibernate proxy or collection.

I'll update this answer when I progress. Currently I'm using solution 2 as a temporary workaround. I'd share the code but it's a mess currently (since I wasn't intending deep-cloning to be the solution).

---

I had a problem with where my relationship was being managed (inverse=true) and also with my session instantiation as it was being injected in. Using that in conjunction with a custom DataContractSurrogate that prevented some of the lazy loading issues now works.

Thanks

var proxy = obj as INHibernateProxy; 
if (proxy != null) 
{ 
    var initializer = proxy.HibernateLazyInitializer; 
    if (initializer.IsUninitialized) 
    { 
        return Activator.CreateInstance(obj.GetType().BaseType); } 
    else 
    { 
        return initializer.GetImplementation(); 
    } 
}