I often create an override in EF for save changes that handles read only and updatable entities

EFSaveChangesOverride.cs

public override int SaveChanges()
{
    var changed = ChangeTracker.Entries()
        .Where(e => e.State == EntityState.Modified || e.State == EntityState.Added);

    foreach (var item in changed)
    {
        if (item.Entity is IReadOnlyEntity)
        {
            item.State = EntityState.Unchanged;
        }
        else if (item.Entity is IUpdateableEntity)
        {
            ((IUpdateableEntity) item).LastUpdated = DateTime.UtcNow;
        }
    }
    return base.SaveChanges();
}