I am trying to transfer my whole project from .net 3.1 to .net 8.
I have resolved some smaller issues but have encountered this major issue which stops me from completing the transition.

What i have in the project is support for both Postgres and SQL Server. Depending on settings in appsettings we register either Postgres or SQL Server database.

        private static void AddDb(this ContainerBuilder builder, ProjectSettings settings)
        {
            switch (settings.DbType)
            {
                case DbTypeEnum.Postgres:
                    {
                        builder.RegisterAssemblyModules(typeof(Postgres.AutofacModule).Assembly);
                        break;
                    }
                case DbTypeEnum.SqlServer:
                    {
                        builder.RegisterAssemblyModules(typeof(SqlServer.AutofacModule).Assembly);
                        break;
                    }
            }
        }

Because of this, i have a parent DB context, and two separate context classes for both Postgres and SQL Server.

In both of those classes there is some custom setup for each of those databases. The main part of the problem is when i am trying to override OnModelCreating to add a custom function for comparison.

In .net 3.1 this code works without a problem, but in .net 8 i am getting an error:

An attempt was made to use the model while it was being created. A DbContext instance cannot be used inside ‘OnModelCreating’ in any way that makes use of the model that is being created.

It seems that in .net 5 they changed how this used to work.

In parent class of DbContext i have this:

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

            modelBuilder.ApplyConfigurationsFromAssembly(Assembly.GetExecutingAssembly());
        }

Code for SQL Server is this:

 public sealed class SqlServerContext : ParentDbContext
 {
     public static string SCHEMA = "dbo";
     public SqlServerContext(DbContextOptions options, ICurrentUser currentUser) : base(options, currentUser)
     {

     }

     protected override void OnModelCreating(ModelBuilder modelBuilder)
     {
         modelBuilder.HasDefaultSchema(SCHEMA);

         base.OnModelCreating(modelBuilder);

         AddCustomFunctions(modelBuilder);
     }

     private void AddCustomFunctions(ModelBuilder modelBuilder)
     {
         MethodInfo likeMethod = typeof(CustomFunctions)
            .GetMethod(nameof(CustomFunctions.Like), new[] { typeof(string), typeof(string) });

         var sqlExpressionFactory = this.GetService<ISqlExpressionFactory>();

         modelBuilder.HasDbFunction(likeMethod).HasTranslation(expressions =>
         {
             var expression = expressions.ElementAt(0); 
             var pattern = expressions.ElementAt(1); 

             return sqlExpressionFactory.Like(expression, pattern);
         });
     }

AddCustomFunctions for Postgres looks like this:

private void AddCustomFunctions(ModelBuilder modelBuilder)
{
    MethodInfo likeMethod = typeof(CustomFunctions)
       .GetMethod(nameof(CustomFunctions.Like), new[] { typeof(string), typeof(string) });

    var sqlExpressionFactory = (NpgsqlSqlExpressionFactory)this.GetService<ISqlExpressionFactory>();

    modelBuilder.HasDbFunction(likeMethod).HasTranslation(expressions =>
    {
        var expression = expressions.ElementAt(0);
        var pattern = expressions.ElementAt(1);

        return sqlExpressionFactory.ILike(expression, pattern);
    });
}

CustomFunctions looks like this:

    public class CustomFunctions
    {
        public static bool Like(string expression, string pattern)
            => throw new InvalidOperationException("this method should not be called client side");
    }

The point of CustomFunctions and mapping this is so throughout the code i can do this for both database types, for example lets say we have a company table:

Context.Companies.FirstOrDefaultAsync(x => CustomFunctions.Like(x.Name, $"%{name}%"));

I have tried to resolve this problem in several ways through Dependency Injection but without success. Trying to pass ISqlExpressionFactory all the way to both Postgres and SQL Server contexts, but no success.
I tried to bring in IServiceProvider or IServiceProviderFactory directly to call the GetService method, but I get an error that they are not registered through DI, which doesn’t make sense because they should be registered during Autofac registration.