Workable.includes(:job_types)
        .where(job_types: {id: nil})

workables.includes(:job_types)
         .where('job_types.id = ?', params[:job_type])
         .references(:job_types)

Workable.includes(:job_types).where(job_types: {id: nil})

workables.includes(:job_types)
         .where(:job_types => {:id => [params[:job_type], nil]})