docs: def _find_prunable_version_ids(self, symbol, keep_mins):
          """
          Find all non-snapshotted versions of a symbol that are older than a version that's at least keep_mins
          minutes old.
  
          Based on documents available on the secondary.
          """
          read_preference = ReadPreference.SECONDARY_PREFERRED if keep_mins > 0 else ReadPreference.PRIMARY
          versions = self._versions.with_options(read_preference=read_preference)
          query = {'symbol': symbol,
                   # Not snapshotted
                   '$or': [{'parent': {'$exists': False}}, {'parent': []}],
                   # At least 'keep_mins' old
                   '_id': {'$lt': bson.ObjectId.from_datetime(dt.utcnow()
                                                              # Add one second as the ObjectId
                                                              # str has random fuzz
                                                              + timedelta(seconds=1)
                                                              - timedelta(minutes=keep_mins)
                                                              )
                           }
                   }
          cursor = versions.find(query,
                                 # Using version number here instead of _id as there's a very unlikely case
                                 # where the versions are created on different hosts or processes at exactly
                                 # the same time.
                                 sort=[('version', pymongo.DESCENDING)],
                                 # Guarantees at least one version is kept
                                 skip=1,
                                 projection={'_id': 1, FW_POINTERS_REFS_KEY: 1, FW_POINTERS_CONFIG_KEY: 1},
                                 )
          return {v['_id']: ([bson.binary.Binary(x) for x in v.get(FW_POINTERS_REFS_KEY, [])], get_fwptr_config(v))
                  for v in cursor}