How to extend DbContext with partial class and partial OnModelCreating method in EntityFramework Core

I'm using EF Core and DatabaseFirst approach. My dbContext is created automatically by Scaffold-DbContext command.

I need to add some new DbSets into a dbContext and add into OnModelCreating method some additional code but after each scaffolding that added code are erased and I have to add it each time again.

What I want to do is to create another partial dbContext class and mark protected override void OnModelCreating(ModelBuilder modelBuilder) method as partial

but get errors:

A partial method cannot have access modifiers or the virtual, abstract, override, new, sealed, or extern modifiers.

A partial method may not have multiple implementing declarations

Here is a pseudo code:

MyDbContext1.cs - generated by Scaffold-DbContext

public partial class MyDbContext : DbContext
{
    public MyDbContext()
    {
    }

    public MyDbContext(DbContextOptions<MyDbContext> options)
        : base(options)
    {
    }

    public virtual DbSet<Client> Clients { get; set; }

    protected override partial void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Client>(entity =>
        {
            // some code ...
        }
    }
}

MyDbContext2.cs - this code I added each time into dbContext after scaffolding:

public partial class MyDbContext
{
    public virtual DbSet<JustAnotherEntity> AnotherEntity { get; set; }

    protected override partial void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<JustAnotherEntity>(entity =>
        {
            entity.HasKey(e => new {e.Id, e.IdAction, e.IdState})
                .ForSqlServerIsClustered(false);
        });
    }
}

EFCore 3 - They FINALLY fixed this!

You can now implement OnModelCreatingPartial in a partial class like this. Note the partial keyword on both the class and method:

public partial class RRStoreContext : DbContext
{
    partial void OnModelCreatingPartial(ModelBuilder builder)
    {
        builder.Entity<RepeatOrderSummaryView>().HasNoKey();
    }
}

If you look at the generated context file - right at the very end of OnModelCreating(...) you'll see...

 OnModelCreatingPartial(modelBuilder);

Note: I use scaffolding, but I needed to manually add HasNoKey for a stored procedure (with a custom return type that wasn't otherwise scaffolded).

An alternative would be creating another context class that inherit from MyDbContext that actually include all the custom code. and then use this new class as your context. This way, there is no need to update the generated code.

public class MyDbContext2 : MyDbContext 
{
    public MyDbContext2()
    {
    }

    public MyDbContext2(DbContextOptions<MyDbContext> options)
        : base(options)
    {
    }

    public virtual DbSet<JustAnotherEntity> AnotherEntity { get; set; }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        base.OnModelCreating(modelBuilder);
        modelBuilder.Entity<JustAnotherEntity>(entity =>
        {
            entity.HasKey(e => new {e.Id, e.IdAction, e.IdState})
                .ForSqlServerIsClustered(false);
        });
    }
}