Support modifying the value of primary or alternate key properties

[tuespetre]

In the following location, an exception is raised when attempting to make modifications to a property that belongs to either a primary or alternate key (using the IsKey extension method):

https://github.com/aspnet/EntityFramework/blob/7.0.0-rc1/src/EntityFramework.Core/ChangeTracking/Internal/InternalEntityEntry.cs#L231

I have to ask: why restrict modifications on alternate keys, and why not use IsPrimaryKey here instead?

[lundcm]

If you're intending on just making a field unique then you can just do this and updates can be made after the initial save. This would be in the DbContext.

    public class ApplicationDbContext : DbContext
    {
        public ApplicationDbContext(DbContextOptions<ApplicationDbContext> options) : base(options) { }

        protected override void OnModelCreating(ModelBuilder builder)
        {
            base.OnModelCreating(builder);

            builder.Entity<ModelName>()
                .HasIndex(b => b.FieldName)
                .IsUnique();
        }
    }

[divega]

Keywords: mutable alternate keys.

[divega]

We discussed mutability of alternate keys once more in the context of #8645. There is a continuum of scenarios we could enable:

1. If they do not act as the principal side of any relationships they can be mutable
2. Else if they are principal but a specific instance does not have any dependency they can be mutated (we would fail if they have dependent in memory and let the database fail if there are dependents that were not loaded in memory)
3. Allow for them to be mutated even when they have dependents. This is more challenging but we believe it can be done with the help of ON UPDATE CASCADE and or translating the UPDATE operation to a sequence of INSERT new principal, UPDATE dependent foreign keys, DELETE all principal.

[ajcvickers]

See also the scenario in #14210