public async Task Upgrade()
{
IReadOnlyCollection <string> existingCollections = await _db.GetCollectionNamesAsync();
if (!existingCollections.Contains(CollectionName))
{
await _db.CreateCollectionAsync(CollectionName);
await _db.GetCollection <RecordedEvent>(CollectionName).CreateIndex("StreamID");
}
string[] missingCollections = _streams
.Select(s => GetStreamInfoName(s))
.Except(existingCollections)
.ToArray();
await Task.WhenAll(missingCollections.Select(x => _db.CreateCollectionAsync(x)));
}