Living in the Tech Avalanche Generation

Archive for March 5th, 2009

Lazy Loading and the Entity Framework. How long I can go on like this?

March 05th, 2009 | Category: Entity Framework, LINQ, ORM

In more recent times I have spent a fair slice of time looking at how to get lazy loading working in the Entity Framework (without using the codeplex solution). Oddly, the driver for this was to get Fetching Strategies implemented for the Entity Framework and not the other way around and I have devoted a quite a few posts recently to that.

If you have followed the story until now, then you will know that I developed a method of transparently lazy loading by using some AOP style trickery and weaving code into the Entities themselves. I did say that I wasn’t going to post on this but it will become apparent why I chose to liberally change my mind about that as we move through this post. Currently the way to have your entity ‘opt’ into lazy loading (with my code), is to decorate it with an attribute in the partial class representation of the Entity itself.

[LazyLoad("Customer", AttributeTargetMembers = @"regex:(Customer$)")]
[LazyLoad("Order_Details", AttributeTargetMembers = @"regex:(Order_Details$)")]
public partial class Order : IFetchable
{
    private IFetchingStrategy _strategy;
    public IFetchingStrategy FetchingStrategy
    {
        get { return _strategy; }
        set { _strategy = value; }
    }
}

Note that this attribute is already written and part of the code download, you don’t have to write any PostSharp attributes yourself to get the code to work. Once having applied the attribute and compiled, PostSharp kicks in and weaves code where it’s directed, in this case the Order entities Customer and Order_Details properties. A look at the code in Reflector confirms this:

Now cool as PostSharp is, this is not a job I was hoping to have to have ever used it for. Entity Framework shouldn’t need that much work for transparent lazy loading and we are promised that will change in a later (next?) version. The lazy loading however is only part of the story. As much as I want transparent Lazy Loading OOTB, I equally need the Eager Loading story to be dealt with. As I have said throughout this series of posts, part of the fetching strategy problem lies in the disconnect between Lazy and Eager fetching and something tells that I will still be rewriting this solution for a later version of Entity Framework.

While we are waiting…..

This solution isn’t proven in battle and needs a lot more work to verify it’s bona fides, but I will push it forward on very small, low risk projects to see how it pans out. I for one really want the Entity Framework to ‘make good’ and reach the hearts and minds of the POCO and DDD audience into the bargain. The question is how long am I prepared to wait and ‘go via the cape’?

Step by Step

Download the code
Copy the LazyLoadAttribute.cs file into your project with an entity model
Reference the PostSharp.Laos, PostSharp.Public libraries.
Reference the Org.TechAvalanche.Orm.Repository library
Decorate the Partial class representation of your Entity with the LazyLoad attribute (copied in the second step) and your done. Write your fetching strategies, Repositories and Specifications and away you go.

I will soon follow with an Item Template for VS.Net that creates the attribute file and makes the references for you automatically.