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 &         


        

Answer 1

# None
Project.joins(:vacancies).group('projects.id').having('count(vacancies) = 0')
# Any
Project.joins(:vacancies).group('projects.id').having('count(vacancies) > 0')
# One
Project.joins(:vacancies).group('projects.id').having('count(vacancies) = 1')
# More than 1
Project.joins(:vacancies).group('projects.id').having('count(vacancies) > 1')

Answer 2

In Rails 4+, you can also use includes or eager_load to get the same answer:

Project.includes(:vacancies).references(:vacancies).
        where.not(vacancies: {id: nil})

Project.eager_load(:vacancies).where.not(vacancies: {id: nil})

Answer 3

You can also use EXISTS with SELECT 1 rather than selecting all the columns from the vacancies table:

Project.where("EXISTS(SELECT 1 from vacancies where projects.id = vacancies.project_id)")

Answer 4

Yeah, vacancies is not a field in the join. I believe you want:

Project.joins(:vacancies).group("projects.id").having("count(vacancies.id)>0")