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

Performing an inner join to the has_many table combined with a group or uniq is potentially very inefficient, and in SQL this would be better implemented as a semi-join that uses EXISTS with a correlated subquery.

This allows the query optimiser to probe the vacancies table to check for the existence of a row with the correct project_id. It doesn't matter whether there is one row or a million that have that project_id.

That's not as straightforward in Rails, but can be achieved with:

Project.where(Vacancies.where("vacancies.project_id = projects.id").exists)

Similarly, find all projects that have no vacancies:

Project.where.not(Vacancies.where("vacancies.project_id = projects.id").exists)

Edit: in recent Rails versions you get a deprecation warning telling you to not to rely on exists being delegated to arel. Fix this with:

Project.where.not(Vacancies.where("vacancies.project_id = projects.id").arel.exists)

Edit: if you're uncomfortable with raw SQL, try:

Project.where.not(Vacancies.where(Vacancy.arel_table[:project_id].eq(Project.arel_table[:id])).arel.exists)

You can make this less messy by adding class methods to hide the use of arel_table, for example:

class Project
  def self.id_column
    arel_table[:id]
  end
end

... so ...

Project.where.not(
  Vacancies.where(
    Vacancy.project_id_column.eq(Project.id_column)
  ).arel.exists
)