docs: def compute_allele_frequencies(self, using=None):
          "Computes the allele frequencies across all samples in this cohort."
          if using is None:
              using = router.db_for_write(self.__class__)
          cursor = connections[using].cursor()
  
          with transition(self, 'Recomputed Allele Frequencies'):
              with transaction.commit_manually(using):
                  # TODO update to use model._meta.db_table..
                  try:
                      # Update count on cohort instance
                      cursor.execute('''
                          UPDATE "cohort" SET "count" = (
                              SELECT COUNT(id)
                              FROM "cohort_sample"
                              WHERE "cohort_id" = "cohort"."id"
                          ) WHERE "id" = %s
                      ''', [self.id])
  
                      # Calculate frequencies for all variants associated with
                      # all samples in this cohort
                      cursor.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"
                          )
                      ''', [self.id])
                      transaction.commit()
                  except DatabaseError, e:
                      transaction.rollback()
                      log.exception(e)
                      raise