外键出现问题并使用EF插入记录

Question

I have the following classes:

public class Instrument
{
    public int Id { get; set; }
    public string Book { get; set; }
    public string Page { get; set; }
}
public class Order
{
    public int Id { get; set; }
    public string OrderName { get; set; }
    public ICollection<MatchedInstrument> MatchedInstruments { get; set; }
}
public class MatchedInstrument
{
    public int Id { get; set; }
    //public int InstrumentId { get; set; }
    public Instrument Instrument { get; set; }
    public bool IsIncluded { get; set; }
    public string Notes { get; set; }
}

And the following EF DbContext:

    public class AppContext : DbContext
{
    public DbSet<Instrument> Instruments { get; set; }
    public DbSet<Order> Orders { get; set; }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Order>().HasKey(o => o.Id)
           .HasMany(o => o.MatchedInstruments)
           .WithMany().Map(m => 
           {
              m.MapLeftKey("orderid");
              m.MapRightKey("selectedmatchid");
              m.ToTable("ordermatchedinstruments");
           });

        modelBuilder.Entity<MatchedInstrument>().HasKey(m => m.Id)
           .HasRequired(m => m.Instrument)
           .WithRequiredPrincipal();
   }
}

Note that the OrderMatchedInstruments table is a join table that simply contains two columns: orderid and matchedinstrumentid (relating to the MatchedInstruments table). The schema for the MatchedInstruments table looks like this:

[dbo].[MatchedInstruments](
    [Id] [int] IDENTITY(1,1) NOT NULL,
    [IsIncluded] [bit] NOT NULL,
    [Notes] [varchar](max) NULL,
    [InstrumentId] [int] NOT NULL

This all seems to work fine for querying data. I am able to query an order and include its matched instruments.

However, when I try to add a new MatchedInstrument to an order, I get an error telling me that InstrumentId cannot be null in the MatchedInstrument table.

...
// Demo Code - count on non-null objects
var instrument = _context.Instruments.FirstOrDefault(); 
var order = _context.Orders.SingleOrDefault(o => o.Id == 5);
order.MatchedInstruments.Add(new MatchedInstrument() { Instrument = instrument });
_context.SaveChanges();    

This leads me to think I need to add an InstrumentId property to the MatchedInstrument class. However, I thought that EF worked fine without having the foreign key as long as it held to the appropriate naming conventions. IOW, the fact that I have navigation property of Instrument would lead me to think that it would automatically look in the table for InstrumentId and would therefore populate that for me.

Is this not the case or am I missing something in regards with how EF deals with foreign keys?

Answer 1

将以下属性添加到模型中的每个ID属性：

[Key, DatabaseGenerated(DatabaseGeneratedOption.Identity)]

Answer 2

I just got this to work. I had set up my relationship incorrectly in my mapping. I changed from using WithRequiredPrincipal to the following:

    modelBuilder.Entity<MatchedInstrument>().HasKey(m => m.Id)
       .HasRequired(m => m.Instrument)
       .WithMany().HasForeignKey(m => m.InstrumentId);

Not sure why I thought I needed to use WithRequiredPrincipal(), but that definitely fixed my issue.

Answer 3

As I see you are not sending the InstrumentId, but the object Instrument, this is the cause of your problem, because you schema has InstrumentId as a Not Null Integer and what you are sending is the entire Instrument Object, you just need to be sure of what data you really need and then change your code.

Grettings...Ing. Jorge Pérez Bárcenas