ohm.js

function(doc) {
  doc.postal_districts.forEach(function(pd) {
    pd.postal_codes.forEach(function(pc) {
      emit(pc.postal_code, {
        postal_name: pc.name,
        postal_code: pc.postal_code,
        type: pc.type,
        postal_district_id: pd.id,
        slug: pd.slug
      })
    });
  });
}

thomas.js

function(doc) {
  pds = doc.postal_districts
  for(i=0;i<pds.length;i++){
    pcs = pds[i].postal_codes
    for(j=0;j<pcs.length;j++){
      data = { postal_name: pcs[j].postal_name, postal_code: pcs[j].postal_code, type: pcs[j].type, postal_district_id: pds[i].id}
      emit(pcs[j].postal_code, data );
    }
  }
}