What's the best method to keep collections updated?

In a folder I have few dozens sub-folders of photos.
Each sub folder is a sub collection under coll://Flickr Photos/myuser.

I found that when I add a photo to one of the existing sub folders it isn't being added to the collection.

I do that operation quite frequently.
What is the best method to keep the folders and the collection in sync one way [folders-->collections]?

Thanks.


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