How to write named scope for sub queries in Active Record Rails

Question

I am trying get unique records from a table based on category column.This query gives those unique records which has been tested on MYSQL. But I wanna know how to write the same thing in Rails as named scope. Can anyone let me know how to write scope for the following query in Rails 5 application:

SELECT * FROM t where id = (select min(id) from t t2 where t2.category = t.category);

Answer 1

I had a same implementation, hope you can get help.

category.rb

class Category < ApplicationRecord
  has_many :category_details
end

category_detail.rb

class CategoryDetail < ApplicationRecord
  belongs_to :category
   scope :get_first_category_vise_details, -> {order(:category_id).group_by(&:category_id).map{|cat_detail_group| cat_detail_group.last.first} }
end

select * from categories;

+----+------+
| id | name |
+----+------+
|  1 | abc  |
|  2 | xyz  |
+----+------+

select * from category_details;

+----+-------------+-------------+
| id | category_id | description |
+----+-------------+-------------+
|  1 |           1 | test        |
|  2 |           1 | test1       |
|  3 |           1 | test2       |
|  4 |           2 | test        |
|  5 |           2 | testabc     |
+----+-------------+-------------+

CategoryDetail.get_first_category_vise_details

[#<CategoryDetail id: 1, category_id: 1, description: "test">, #<CategoryDetail id: 4, category_id: 2, description: "test">]