Make InMemory provider enforce uniqueness of Alternate Keys

[philippecp]

When adding two entities with the same alternate key in an InMemory database, no exception is thrown and both entities are added.

[rowanmiller]

Agreed that this would be a useful thing to add to the in-memory provider. We aren't planning to do this for initial RTM. Marking as up for grabs as this would make a good contribution too.

[android2221]

Is this issue still relevant? I'd like to try my hand at it.

[android2221]

With a cursory glance / test I was generating errors like this when adding an object with the same alternate key:
The instance of entity type 'Customer' cannot be tracked because another instance with the same key value for {'MyFakeAlternatekey'} is already being tracked. When attaching existing entities....

I got this when adding the second entity, before calling save.

This would be the ideal output right? If so, we might be able to close this one.

[AndriySvyryd]

@android2221 Here's a test for this:

[ConditionalFact]
public void Throws_on_duplicate_alternate_key_values()
{
    var modelBuilder = InMemoryTestHelpers.Instance.CreateConventionBuilder();
    modelBuilder.Entity<MyEntityType>(
        eb =>
        {
            eb.HasAlternateKey("AlternateId");
        });

    var optionsBuilder = new DbContextOptionsBuilder()
        .UseModel(modelBuilder.FinalizeModel())
        .UseInMemoryDatabase("AlternateKeys");

    using (var context = new DbContext(optionsBuilder.Options))
    {
        context.Add(new MyEntityType { Id = 1, AlternateId = 11});
        context.SaveChanges();
    }

    using (var context = new DbContext(optionsBuilder.Options))
    {
        context.Add(new MyEntityType { Id = 2, AlternateId = 11 });
        Assert.Throws<DbUpdateException>(() => context.SaveChanges());
    }
}

private class MyEntityType
{
    public int Id { get; set; }
    public int AlternateId { get; set; }
}

[android2221]

@AndriySvyryd does this pass? In my testing I did not save the context between additions. If it passes, then we can close the issue right? If not, I'll dig deeper :)

[AndriySvyryd]

@android2221 It doesn't pass

[android2221]

This one is outside my wheelhouse, I give up.

[ajcvickers]

We recommend against using the in-memory provider for testing--see Testing EF Core Applications. While we have no plans to remove the in-memory provider, we will not be adding any new features to this provider because we believe valuable development time is better spent in other areas. When feasible, we plan to still fix regressions in existing behavior.