I managed to do it this way, if someone has a better approach, I'm happy for any hints 🙂

let users = await prisma.user.findMany({
    select: {
      id: true,
      name: true,
      solves: {
        select: {
          challenge: true,
          createdAt: true,
        },
      },
    },
  });

  users = users.map((u) => {
    const sum = u.solves.reduce(
      (previous, current) => previous + current.challenge.points,
      0
    );
    return { ...u, points: sum };
  });