EF Core Add function returns negative id

问题

I noticed a weird thing today when I tried to save an entity and return its id in EF Core

Before: After:

I was thinking about if it was before calling saveChanges() but it works with another entity with a similar setup.

ps: I use unit of work to save all changes at the end.

What was the reason?

回答1:

It will be negative until you save your changes. Just call Save on the context.

_dbContext.Locations.Add(location);
_dbContext.Save();

After the save, you will have the ID which is in the database. You can use transactions, which you can roll back in case there's a problem after you get the ID.

The other way would be not to use the database's built-in IDENTITY fields, but rather implement them yourself. This can be very useful when you have a lot of bulk insert operations, but it comes with a price — it's not easy to implement.

回答2:

Apparently, this isn't a bug it's a feature: https://github.com/aspnet/EntityFrameworkCore/issues/6147

It's not too arduous to override this behavior like so:

 public class IntValueGenerator : TemporaryNumberValueGenerator<int>
 {
     private int _current = 0;

     public override int Next(EntityEntry entry)
     {
         return Interlocked.Increment(ref _current);
     }    
 }

Then reference the custom value generator here:

public class CustomContext: DbContext
{
    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        foreach (var type in modelBuilder.Model.GetEntityTypes().Select(c => c.ClrType))
        {
            modelBuilder.Entity(type, b =>
            {              
                b.Property("Id").HasValueGenerator<IntValueGenerator>();
            });
        }
        base.OnModelCreating(modelBuilder);
    }
}

This will result in temporary Id's that increment from "1", however, more effort is required to prevent key conflicts when attempting to save.

来源：https://stackoverflow.com/questions/41732585/ef-core-add-function-returns-negative-id