sql - Find all records which have a count of an association greater than zero

Question

I'm trying to do something that I thought it would be simple but it seems not to be.

I have a project model that has many vacancies.

class Project < ActiveRecord::Base

  has_many :vacancies, :dependent => :destroy

end

I want to get all the projects that have at least 1 vacancy. I tried something like this:

Project.joins(:vacancies).where('count(vacancies) > 0')

but it says

SQLite3::SQLException: no such column: vacancies: SELECT "projects".* FROM "projects" INNER JOIN "vacancies" ON "vacancies"."project_id" = "projects"."id" WHERE ("projects"."deleted_at" IS NULL) AND (count(vacancies) > 0).

Answer 1

1) To get Projects with at least 1 vacancy:

Project.joins(:vacancies).group('projects.id')

2) To get Projects with more than 1 vacancy:

Project.joins(:vacancies).group('projects.id').having('count(project_id) > 1')

3) Or, if Vacancy model sets counter cache:

belongs_to :project, counter_cache: true

then this will work, too:

Project.where('vacancies_count > ?', 1)

Inflection rule for vacancy may need to be specified manually?