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Tip 40 – How to materialize presentation models via L2E

  • Artigo

Problem:

Imagine that you have these entities

public class Product
{
public int ID { get; set; }
public string Name { get; set; }
public virtual Category Category { get; set; }
}

public class Category
{
public int ID { get; set; }
public string Name { get; set; }
public virtual List<Product> Products { get; set; }
}

But for your UI you want to display the product id, product name and the product’s category’s name.

You might try passing this query to the presentation layer.

var displayData = from product in ctx.Products.Include("Category")
select product;

But then you are passing more than is required around, and binding the UI tightly to the conceptual model, which is probably a bad idea.

Your next try might be a query like this:

var displayData = from product in ctx.Products
select new {
ID = product.ID,
Name = product.Name,
CategoryName = product.Category.Name
};

But then you’ll quickly discover you can’t pass the anonymous types to another method, at least not without a nasty hack.

Solution: 

Most people think LINQ to Entities can only materialize Entities and Anonymous types.

But in fact it turns out it can materialize any non-generic class that has a default constructor.

Anyway what this means is you can create a view class like this:

public class ProductView
{
public int ID { get; set; }
public string Name { get; set; }
public string CategoryName { get; set; }
}

And then write this:

var displayData = from product in ctx.Products
select new ProductView {
ID = product.ID,
Name = product.Name,
CategoryName = product.Category.Name
};

And then pass this to your view without any problems at all.

I only just found out about this myself, I had always assumed this would fail.

So this was a nice pleasant surprise.

Comments

  • Anonymous
    November 06, 2009
    I have several examples of projection onto Presentation Models (plus some thought into how to handle data coming in) in this presentation:http://blogs.teamb.com/craigstuntz/2009/09/11/38394/I'm thinking of putting up a demo app for using repositories, presentation models, and the EF with ASP.NET MVC.
  • Anonymous
    November 07, 2009
    @Craig,Cool. Thanks for the link, I'm sure people reading my blog will find it interesting. Let me know if/when you do the demo app, I will definitely link to it.Alex
  • Anonymous
    December 01, 2009
    Yes, I do. Great presentation Craig! yeah, hope to see your demo too!
  • Anonymous
    December 31, 2009
    A demo app is in the works, but for the time being here's a post which elaborates on the above:http://blogs.teamb.com/craigstuntz/2009/12/31/38500/
  • Anonymous
    May 05, 2010
    The comment has been removed