Rails order by results count of has_many association

If you expect to use this query frequently, I suggest you to use built-in counter_cache

# Job Model
class Job < ActiveRecord::Base
  belongs_to :company, counter_cache: true
  # ...
end

# add a migration
add_column :company, :jobs_count, :integer, default: 0

# Company model
class Company < ActiveRecord::Base
  scope :featured, order('jobs_count DESC')
  # ...
end

and then use it like

@featured_company = Company.featured

Rails 5+

Support for left outer joins was introduced in Rails 5 so you can use an outer join instead of using counter_cache to do this. This way you'll still keep the records that have 0 relationships:

Company
  .left_joins(:jobs)
  .group(:id)
  .order('COUNT(jobs.id) DESC')
  .limit(10)

---

The SQL equivalent of the query is this (got by calling .to_sql on it):

SELECT "companies".* FROM "companies" LEFT OUTER JOIN "jobs" ON "jobs"."company_id" = "companies"."id" GROUP BY "company"."id" ORDER BY COUNT(jobs.id) DESC

Something like:

Company.joins(:jobs).group("jobs.company_id").order("count(jobs.company_id) desc")