mb.Entity<Question>()
                .HasMany(q => q.Variants)
                .WithOne()
                .HasForeignKey("QuestionId")
                .IsRequired()
                .OnDelete(DeleteBehavior.Cascade);

            mb.Entity<Question>()
                .HasOne(typeof(Variant))
                .WithOne()
                .HasForeignKey<Question>("AnsweredVariantId")
                .IsRequired(false)
                .OnDelete(DeleteBehavior.Restrict);

            // EF creates Unique Index for nullable fields
            mb.Entity<Question>()
                .HasIndex(q => q.AnsweredVariantId)
                .IsUnique(false);

            // create index instead of FK hence the cyclic dependency between Question and Variant
            mb.Entity<Question>()
                .HasIndex(q => q.CorrectVariantId)
                .IsUnique();