Why do it with Ruby, when 63 lines of SQL would also work :)

points_breakdown.sql

CREATE VIEW points_breakdowns AS
WITH users_tasks AS (
  SELECT
    users.id AS user_id,
    tasks.id AS task_id,
    GREATEST(COALESCE(points + adjustment, 0), 0) AS points
  FROM users
  INNER JOIN tasks ON TRUE
  LEFT JOIN solutions ON (solutions.task_id = tasks.id AND solutions.user_id = users.id)
  ORDER BY users.id, tasks.id
), tasks_summary AS (
  SELECT
    user_id,
    array_agg(points) AS breakdown,
    SUM(points) AS points
  FROM users_tasks
  GROUP BY user_id
), users_quizzes AS (
  SELECT
    users.id AS user_id,
    quizzes.id AS quiz_id,
    COALESCE(points, 0) AS points
  FROM users
  INNER JOIN quizzes ON TRUE
  LEFT JOIN quiz_results ON (quiz_results.quiz_id = quizzes.id AND quiz_results.user_id = users.id)
  ORDER BY users.id, quizzes.id
), quizzes_summary AS (
  SELECT
    user_id,
    array_agg(points) AS breakdown,
    SUM(points) AS points
  FROM users_quizzes
  GROUP BY user_id
), stars_summary AS (
  SELECT
    users.id AS user_id,
    COUNT(posts.starred) AS points
  FROM users
  LEFT JOIN posts ON posts.user_id = users.id AND posts.starred
  GROUP BY users.id
), vouchers_summary AS (
  SELECT
    users.id AS user_id,
    COUNT(vouchers.id) AS points
  FROM users
  LEFT JOIN vouchers ON vouchers.user_id = users.id
  GROUP BY users.id
), arrays_summary AS (
  SELECT
    users.id AS user_id,
    COALESCE(tasks_summary.breakdown, '{}') AS tasks_breakdown,
    COALESCE(tasks_summary.points, 0) AS tasks,
    COALESCE(quizzes_summary.breakdown, '{}') AS quizzes_breakdown,
    COALESCE(quizzes_summary.points, 0) AS quizzes
  FROM users
  LEFT JOIN tasks_summary ON tasks_summary.user_id = users.id
  LEFT JOIN quizzes_summary ON quizzes_summary.user_id = users.id
)
SELECT
  rank() OVER (ORDER BY arrays_summary.tasks + arrays_summary.quizzes + stars_summary.points + vouchers_summary.points DESC) AS rank,
  users.id AS id,
  users.name AS name,
  users.faculty_number AS faculty_number,
  arrays_summary.tasks_breakdown AS tasks_breakdown,
  arrays_summary.tasks AS tasks,
  arrays_summary.quizzes_breakdown AS quizzes_breakdown,
  arrays_summary.quizzes AS quizzes,
  stars_summary.points AS stars,
  vouchers_summary.points AS vouchers,
  arrays_summary.tasks + quizzes_summary.points + stars_summary.points + vouchers_summary.points AS total
FROM users
LEFT JOIN arrays_summary ON arrays_summary.user_id = users.id
LEFT JOIN quizzes_summary ON quizzes_summary.user_id = users.id
LEFT JOIN stars_summary ON stars_summary.user_id = users.id
LEFT JOIN vouchers_summary ON vouchers_summary.user_id = users.id
WHERE NOT users.admin
ORDER BY rank