EF 6 - How to solve foreign key error for composite key?

Question

I'm playing with this problem for days and can't find any solution.

I'm using code first strategy, .NET MVC 4.5 and EF 6

I've two models with composite keys:

public class Category : DbContext
{
    [Key, Column(Order = 0),DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int CategoryId { get; set; }

    [Key, Column(Order = 1)]
    public int ShopId { get; set; }

    public string Name { get; set; }  
}

public class Product : DbContext
{
  [Key, Column(Order = 0),DatabaseGenerated(DatabaseGeneratedOption.Identity)]
  public int ProductId { get; set; }

  [Key, Column(Order = 1)]
  public int ShopId { get; set; }

  public int CategoryId { get; set; }

  [ForeignKey("CategoryId, ShopId")]        
  public virtual Category Category { get; set; }
}

when I going to run add-migration command I get this code in the migration folder:

 CreateTable(
  "dbo.Categories",
   c => new
   {
    CategoryId = c.Int(nullable: false, identity: true),
    ShopId = c.Int(nullable: false),
    Name = c.String(nullable: false, maxLength: 5)                  
   })
   .PrimaryKey(t => new { t.CategoryId, t.ShopId });

   CreateTable(
     "dbo.Products",
      c => new
      {
        ProductId = c.Int(nullable: false, identity: true),
        ShopId = c.Int(nullable: false),
        CategoryId = c.Int(nullable: false)
      })
      PrimaryKey(t => new { t.ProductId, t.ShopId })
      .ForeignKey("dbo.Categories", t => new { t.CategoryId, t.ShopId }, cascadeDelete: true)               
      .Index(t => new { t.ShopId, t.CategoryId });

then, when I run update-database command everything is OK until first accees to the database via EF. I'll get this error:

Foreign key constraint 'Product_Category' from table Product (ShopId, CategoryId) to table Category (CategoryId, ShopId):: Insufficient mapping: Foreign key must be mapped to some AssociationSet or EntitySets participating in a foreign key association on the conceptual side.

What I now:

1. If I delete foreign key from Model class, EF still generate foreign key to the migration code. Problem persist.

2. If I delete foreign key from generated code for migration. Problem still persist. (even if in DB is not any foreign key between tables)

3. If I change in Product class attribute CategoryId from int to string, EF will generate new two columns to the table Product_CategoryId and Product_ShopId and put foreign key on these columns. Problem solved....

Really don't understant where's problem. Of course I don't want to have these two columns in table. I wanna have direct foreign key from CategoryId and ShopId columns.

Really thx. for any advice.

Answer 1

From your code, is there any specific reason why a product Or Category should inherit from DbContext ? Also, try the below and it will work. The reason i am suspecting why it failed is "A part of the Composite Primary Key (in Product) is also a part of Foreign Key (Composite Fk) from Category"

public class SampleContext : DbContext
    {
        public IDbSet<Category> Categories { get; set; }
        public IDbSet<Product> Products { get; set; }
        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {
             base.OnModelCreating(modelBuilder);
        }
    }
    public class Category
    {
        [Key, Column(Order = 0), DatabaseGenerated(DatabaseGeneratedOption.Identity)]
        public int CategoryId { get; set; }
        [Key, Column(Order = 1)]
        public int ShopId { get; set; }
        public string Name { get; set; }
    }
    public class Product 
    {
        [Key, Column(Order = 0), DatabaseGenerated(DatabaseGeneratedOption.Identity)]
        public int ProductId { get; set; }
        public int ShopId { get; set; }
        public int CategoryId { get; set; }
        [ForeignKey("CategoryId, ShopId")]
        public virtual Category Category { get; set; }
    }

Let me know if this what you are expecting. and First thing at place is EF Migration Runner should throw exception ????