将数据去规范化为代码第一个实体框架实体，没有SQL视图

Question

First, here's a simple example database model, which has Products assigned to Categories , where CategoryId in Products is the FK relationship to Categories .

Products :

• ProductId (PK), INT
• ProductName VARCHAR(255)
• CategoryId (FK), INT

Categories

• CategoryId (PK), INT
• CategoryName VARCHAR(255)

For the .NET application data model, only a de-normalized representation of a Product is defined as an entity class:

public class Product
{
    public int ProductId { get; set; }
    public string ProductName { get; set; }
    public int CategoryId { get; set; }
    public string CategoryName { get; set; }
}

There is no Category class defined, and for this example, none is planned.

In the code-first Entity Framework DbContext -derived class, I've setup the DbSet<Product> Products entity set:

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

And in the EntityTypeConfiguration , I'm attempting to wire it up, but I'm just not able to get it working right:

public class ProductConfiguration : EntityTypeConfiguration<Product>
{
    public ProductConfiguration()
    {
        HasKey(t => t.ProductId);

        // How do I instruct EF to pull just the column 'CategoryName'
        // from the FK-related Categories table?
    }
}

I realize that a SQL View could be created and then I could tell EF to map to that view using ToTable("App1ProductsView") , but in this example, I'd like to avoid doing so.

In a SQL ADO.NET ORM solution, there's no issue here. I can simply write my own SQL statement to perform the INNER JOIN Categories c ON c.CategoryId = p.CategoryId join. How can I use the EF code-first Fluent API to perform this same inner join when populating the entity?

In my research, I've seen a lot of "entity split across multiple tables" topics, but this is not that. Categories and Products are two distinct entities (from a database perspective), but the .NET code is meant to stay unaware of that.

Failed Attempt 1 :

This does not work, and produces a strange query (seen with SQL Server Profiler).

Fluent config:

Map(m =>
{
    m.Property(t => t.CategoryName);
    m.ToTable("Categories");
});

Resulting SQL:

SELECT 
    [Extent1].[ProductId] AS [ProductId], 
    [Extent2].[ProductName] AS [ProductName], 
    [Extent2].[CategoryId] AS [CategoryId], 
    [Extent1].[CategoryName] AS [CategoryName], 
FROM  [dbo].[Categories] AS [Extent1]
INNER JOIN [dbo].[Product1] AS [Extent2] ON [Extent1].[ProductId] = [Extent2].[ProductId]

Answer 1

Short answer: MS EF6 is not designed for that, you can't do it easily.

Please read about the keys , relationships and how to configure one-to-many relationship .

EF expects your entity to have a key which is a part of a mapped table. You could use splitting, ie put some properties in Table1 and some to Table2, but only if both tables share the same primary key. That works for [1] -> [0..1] relationships only. What you have is a one-to-many.

EF way of mapping your db schema is to create two entities and access the category name as Product.Category.Name .

If you completely do not want to expose Category entity, you may use an internal class and a protected property, exposing category name as a sql-ignored property public string CategoryName => this.Category?.Name .

The other option is to use untracked SqlQuery . Then you will have to write SQL query yourself, as you did for the pure ADO.NET solution.

If you don't want to use EF change tracking, relationships and etc, consider a lighter ORM like Dapper, linq2db or BLToolkit.