Entity Framework query not generated - asp.net-mvc

Question

I have the following object model:
[Table("APA_QuestionProduct")]
public class QuestionProduct
{
[Key, ForeignKey("Question"), Column(Order=0)]
public int QuestionID { get; set; }
[ForeignKey("QuestionID")]
public Question Question { get; set; }
[Key, ForeignKey("Product"), Column(Order=1)]
public int ProductID { get; set; }
[ForeignKey("ProductID")]
public Product Product { get; set; }
}
Table:
USE [qbm]
GO
/****** Object: Table [dbo].[APA_QuestionProduct] Script Date: 5/21/2013 6:52:46 PM ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[APA_QuestionProduct](
[QuestionID] [int] NOT NULL,
[ProductID] [int] NOT NULL,
CONSTRAINT [PK_APA_QuestionProduct] PRIMARY KEY CLUSTERED
(
[QuestionID] ASC,
[ProductID] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
ALTER TABLE [dbo].[APA_QuestionProduct] WITH CHECK ADD CONSTRAINT [FK_APA_QuestionProduct_APA_Product] FOREIGN KEY([ProductID])
REFERENCES [dbo].[APA_Product] ([ProductID])
GO
ALTER TABLE [dbo].[APA_QuestionProduct] CHECK CONSTRAINT [FK_APA_QuestionProduct_APA_Product]
GO
ALTER TABLE [dbo].[APA_QuestionProduct] WITH CHECK ADD CONSTRAINT [FK_APA_QuestionProduct_APA_Question] FOREIGN KEY([QuestionID])
REFERENCES [dbo].[APA_Question] ([QuestionID])
GO
ALTER TABLE [dbo].[APA_QuestionProduct] CHECK CONSTRAINT [FK_APA_QuestionProduct_APA_Question]
GO
so the table behind has only 2 foreign keys (that are also primary keys). My question object has a list of products. When I update the ProductID foreign key and call 'SaveChanges' on the context no query/update is generated in db:
question.Products[1].ProductID = 4;
db.Entry(question.Products[1]).State = EntityState.Modified;
db.SaveChanges();
I looked in InteliTrace to check the query but no query is called over my QuestionProduct table even if my QuestionProduct object is changed. Why the table is not updated? No error is thrown.

Answer 1

Your entity QuestionProduct only contains key properties, no other scalar properties. Entity Framework does not allow to change (primary) key properties.
You must delete the old link record and create a new one to establish the new relationship, for example like so:
QuestionProduct oldProduct = question.Products[1];
QuestionProduct newProduct = new QuestionProduct
{
QuestionID = question.QuestionID,
ProductID = 4
};
db.QuestionProducts.Attach(oldProduct);
db.QuestionProducts.Remove(oldProduct);
db.QuestionProducts.Add(newProduct);
db.SaveChanges();
But you should really model this as a many-to-many relationship. You don't need the QuestionProduct entity and could have collections directly refering from Question to Product and vice versa without having to navigate through an intermediate entity.
It is shown here for example how it works.