Rails select by number of associated records

Question

I have following models in my rails app:

class Student < ApplicationRecord
  has_many :tickets, dependent: :destroy
  has_and_belongs_to_many :articles, dependent: :destroy

class Article < ApplicationRecord
  has_and_belongs_to_many :students, dependent: :destroy

class Ticket < ApplicationRecord
  belongs_to :student, touch: true

I need to extract all Students who has less than articles and I need to extract all Students who's last ticket title is 'Something'.

Everything I tried so far takes a lot of time. I tried mapping and looping through all Students. But I guess what I need is a joined request. I am looking for the most efficient way to do it, as database I am working with is quite large.

Answer 1

You asked "I need to extract all Students who has less than articles" . I'll presume you meant "I need to extract all Students who have less than X articles" . In that case, you want group and having https://guides.rubyonrails.org/active_record_querying.html#group .

For example, Article.group(:student_id).having('count(articles.id) > X').pluck(:student_id) .

To address your second question, you can use eager loading https://guides.rubyonrails.org/active_record_querying.html#eager-loading-associations to speed up your code.

result = students.filter do |student|
  students.tickets.last.name == 'Something'
end

Answer 2

go with @MCI's answer for your first question. But a filter/select/find_all or whatever (although I havn't heared about filter method in ruby) through students record takes n queries where n is the number of student records (called an n+1 query).

studs = Student.find_by_sql(%{select tmp.id from (
  select student_id as id from tickets where name='Something' order by tickets.created_at desc
) tmp group by tmp.id})

Answer 3

Here association is HABTM so below query should work

x = 10
Student.joins(:articles).group("articles_students.student_id").having("count(articles.id) < ?",x)