docs: def forwards(self, orm):
          "Write your forwards methods here."
          db_table = orm['samples.CohortVariant']._meta.db_table
          db.execute('TRUNCATE TABLE "{0}"'.format(db_table))
          db.execute('ALTER SEQUENCE "{0}_id_seq" RESTART 1'.format(db_table))
          cohorts = list(orm['samples.Cohort'].objects.all())
  
          for c in cohorts:
              # Calculate frequencies for all variants associated with all
              # samples in this cohort
              db.execute('''
                  INSERT INTO cohort_variant (cohort_id, variant_id, af) (
                      SELECT c.id, r.variant_id, COUNT(r.id) / c."count"::float
                      FROM sample_result r
                          INNER JOIN sample s ON (r.sample_id = s.id)
                          INNER JOIN cohort_sample cs ON (cs.sample_id = s.id)
                          INNER JOIN cohort c ON (cs.cohort_id = c.id)
                      WHERE c.id = %s
                      GROUP BY c.id, r.variant_id, c."count"
                  )
              ''', [c.pk])