导轨左外连接与行，包括 NULL

Question

I need to do a complex outer join. I wanna have a row in the result for the service interval if there is no service_note that's connected to it. As you see from the query service_interval belongs to one service_job which is connected to service_notes through the joint table service_note_jobs . At the moment it returns an empty relation if there is no service_note connected to that service_interval .

def self.distance_and_time_until_next_service(service_interval_ids)
  ServiceInterval
    .left_joins(vehicle: { service_notes: :service_note_jobs })
    .where(
      'service_note_jobs.service_job_id = service_intervals.service_job_id AND '\
      'service_intervals.vehicle_id = service_notes.vehicle_id'
    )
    .where(service_intervals: { id: service_interval_ids })
    .group('service_intervals.id')
    .select(
      'service_intervals.id',
      "MAX(GREATEST(service_notes.odometer, service_intervals.created_odometer) - vehicles.odometer + service_intervals.distance_interval) AS distance_left_until_next_service",
      "MAX(ROUND(EXTRACT(epoch FROM(GREATEST(service_notes.date, service_intervals.created_at) - NOW())) / 86400  + service_intervals.time_interval)) AS time_left_until_next_service"
    )
end

Answer 1

Try to add this after your left_joins:

.where(vehicle: { service_notes: { id: nil } })

Or more general sample

User.left_joins(:posts).where(posts: { id: nil })

This will give you users with no posts.