How to search group_by with association in Rails - ruby-on-rails

Question

I don't know how I describe this question, at first I want show my model which maintaining a relation like below
category.rb
class Category < ApplicationRecord
has_many :job_categories, dependent: :destroy
has_many :jobs, through: :job_categories
end
job.rb
class Job < ApplicationRecord
has_many :job_categories, dependent: :destroy
has_many :categories, through: :job_categories
end
job_category.rb
class JobCategory < ApplicationRecord
belongs_to :category, counter_cache: :jobs_count
belongs_to :job
end
schema.rb
create_table "categories", force: :cascade do |t|
t.string "name"
t.string "parent"
end
the parent is a column which maintain the group like Technology and under this ruby,rails,programming etc which is Technology related.
Below is my query for showing group by category
Category.select(:id, :name, :parent).group_by{|p| p.parent}
and it's showing like this
Technology
ruby
rails
etc
Now I want to show all jobs in group by Technology, I have a query for this like
Job.joins(:categories).where('lower(categories.parent) LIKE lower(?)', "%#{params[:parent]}%")
and it's showing wrong output like if I have only one job which categories is ruby,rails then this one job is showing two times, one for ruby and one for rails.
Thanks

Answer 1

Your associations are correct, you can retrieve all unique jobs for some categories by following:
Job.joins(:job_categories).joins(:categories).where('lower(categories.parent) LIKE lower(?)', "%#{params[:parent]}%").distinct
This will join the jobs with the intermediate table job_categories and jobs on relevant keys and where clause will then allow you to be selective on what you want to retrieve.
SELECT DISTINCT "jobs" .*
FROM "jobs" INNER
JOIN "job_categories" ON "job_categories" ."job_id" = "jobs" ."id" INNER
JOIN "job_categories" "job_categories_jobs_join" ON "job_categories_jobs_join" ."job_id" = "jobs" ."id" INNER
JOIN "categories" ON "categories" ."id" = "job_categories_jobs_join" ."category_id"
WHERE
(
lower ( categories.parent ) LIKE lower ( "Technology" ) )
Update:
Actually, we don't need to have explicit join to job_categories either, the following should suffice:
Job.joins(:categories).where('lower(categories.parent) LIKE lower(?)', "%#{params[:parent]}%").distinct
SELECT DISTINCT "jobs".* FROM "jobs" INNER JOIN "job_categories" ON "job_categories"."job_id" = "jobs"."id" INNER JOIN "categories" ON "categories"."id" = "job_categories"."category_id" WHERE (lower ( categories.parent ) LIKE lower ( "Technology" ))

Answer 2

Just few other options to fetch and group records with association has_many_through:
# Filtering by query
Job.joins(:categories).select('jobs.id, jobs.name, categories.parent').where('lower(categories.parent) LIKE lower(?)', "Technology").distinct.inspect
# => #<ActiveRecord::Relation [#<Job id: 1, name: "Developer">, #<Job id: 2, name: "Debugger">]>
# Grouping by categories.parent, return a hash
Job.joins(:categories).select('jobs.id, jobs.name, categories.parent').all.distinct.group_by(&:parent)
# => {"Technology"=>[#<Job id: 1, name: "Developer">, #<Job id: 2, name: "Debugger">], "Mechanics"=>[#<Job id: 3, name: "Technic">]}
# Accessing the hash by key
Job.joins(:categories).select('jobs.id, jobs.name, categories.parent').all.distinct.group_by(&:parent)["Technology"]
#=> [#<Job id: 1, name: "Developer">, #<Job id: 2, name: "Debugger">]