How to Rank in Rails After Grouping - ruby-on-rails

Question

In my Rails 6 app where I'm using Postgres, I have a table called UserCategories.
| id | user_id | category_id | points| rank |
What I'm trying to do is:
Group the records by category_id
Sort the records for each category_id by points (desc)
Update the rank field based on the order the record is in for the category_id
Example (desired rank determined by the points per category_id):
| id | user_id | category_id | points| rank |
| 1 | 1 | 1 | 2 | | # I want rank to be 1
| 2 | 2 | 1 | 1 | | # I want rank to be 2
| 3 | 1 | 2 | 3 | | # I want rank to be 1
| 4 | 2 | 2 | 3 | | # I want rank to be 1
My model method:
def self.calculate_user_category_ranks
#user_categories = UserCategory.select(:id, :points, :user_id, :category_id, :rank).all.order(points: :desc).group_by(&:category_id)
# returns:
# {2=>[#<UserCategory:0x000000000de8be00 id: 2, user_id: 1, category_id: 2, points: 3, rank: 0>, #<UserLeague:0x000000000de8bce8 id: 4, user_id: 2, category_id: 2, points: 3, rank: 0>],
1=>[#<UserCategory:0x000000000de8bbf8 id: 1, user_id: 1, category_id: 1, points: 2, rank: 0>, <UserLeague:0x000000000de8bb30 id: 3, user_id: 2, category_id: 1, points: 1, rank: 0>]}
rank = 0
points_counter = 0
#user_categories.each do |id, points|
uc = UserCategory.find(id)
if points != point_counter
rank += 1
point_counter = points
end
uc.rank = rank
uc.save
end
end
After I execute this code:
| id | user_id | category_id | points| rank |
| 1 | 1 | 1 | 2 | 2 | # I want rank to be 1
| 2 | 2 | 1 | 1 | 0 | # I want rank to be 2
| 3 | 1 | 2 | 3 | 1 | # I want rank to be 1
| 4 | 2 | 2 | 3 | 0 | # I want rank to be 1
Can someone please help me determine what I'm doing wrong?

Answer 1

You may be choosing to put rank as a database column for reasons of efficiency, but the principles of database normalization would suggest that it is "bad practice" to have a column whose value is calculable from the other columns in the table. So, recognizing that you may not accept this solution for reasons of efficiency, let me suggest that, for any particular instance of UserCategory you can determine its rank in Ruby:
class UserCategory < ApplicationRecord
scope :in_the_same_category, ->(category_id) { where("category_id = ?", category_id }
def in_my_category
UserCategory.in_the_same_category(category_id)
end
def rank
in_my_category.
sort_by(&:points).
reverse.
map(&:points).
uniq.
index(points) + 1
end
end

Answer 2

Start with
UserCategory.group(:category_id)
Then build it up
If you really need to restrict the fields returned or add a where clause then
UserCategory.where(some_field: "some_value").select(:id, :points, :user_id, :category_id, :rank).group(:category_id).order(:points :desc)
https://apidock.com/rails/ActiveRecord/QueryMethods/group for more info