EntityFramework自定义SQL查询的虚拟属性

Question

We use 6.1.2 EF and we have edmx generated from database.

We have POCO class like:

public partial class Category
{
    public Category()
    {
        this.Products = new HashSet<Product>();
    }

    public long CategoryID { get; set; }
    public string Name { get; set; }

    public virtual ICollection<Product> Products { get; set; }
}

What I need to do, is insert new virtual property with custom sql query calling like:

public virtual int ProductsCount { get; set; }

which will return select count(1) from dbo.Product where CatgoryID = *** Is it possible to define virtual property like this? I find some sql query definitions in .edmx xml file, so I hope, there can be defined mine too. What can I replace the *** with?

After all, my code should be called as simple as:

context
    .Category
    .Include(x => x.ProductsCount)
    .ToList();

Answer 1

Messing with the generated XML is genrally not a good idea. Any regeneration will override your code.

IMO, your property shall look like :

public int ProductCount { get { return Products.Count(); } }

And your request should be :

context.Category.Include(x => x.Products).ToList();

If loading the full products list might lead to problems, I would use a DB View ot the Category table with a computed column and then update my EF mapping to have this new entity inherit the Category entity.

Answer 2

If Entity Framework had formula properties, like NHibernate, this could be a viable option. However, with Entity Framework you can't do this without breaking persistence ignorance and generating n + 1 queries.

• Persistence ignorance: entities are not supposed to have any reference to the data layer that created them. This is an important proposition that shouldn't be broken. Else you create a situation in which it is hard to track (and to predict) when and where queries will be executed. Moreover, it breaks the single responsibility principle.

• n + 1 anti pattern: other than NHibernate with formula properties, EF won't be able to integrate the embedded query in the query it executes to fetch Category s from the database (1). Each category will subsequently execute its own query (n).

The best way to do this is project the data to a view model (or DTO class, whichever name suits you best):

context.Categories.Select(c => new CategoryModel
{
    Name = c.Name,
    OtherProperty = c.OtherProperty,
    ProductsCount = c.Products.Count()
}

When it comes to projections you should always look at what AutoMapper can do for you. If you have this CategoryModel class you can define a mapping once (at application startup)...

Mapper.CreateMap<Catecory,CategoryModel>();

...and use it like so:

context.Categories.Project().To<CategoryModel>();

The nice thing of AutoMapper is that it parses a property like ProductsCount into a property in the source class, Products , and the operation working on it, Count() . This single line of code does exactly the same thing as the previous code snippet.

Side note: likewise, AutoMapper parses nested properties. A property CategoryName you might have in a ProductModel class (if Product-Category is n-1) would resolve to product.Category.Name .