Whoosh index file overwritten when I open it to add new documents

Question

I have a problem with Whoosh. I want to create an index in different moments, because the query to extract data is heavy. I fixed almost all the problems, but I can't get over the problem that every time I reopen the index to add new documents, the file is cleaned instead of simply adding new documents. I tried to use update_document instead of add_document, and FileStorage.open_index instead of index.open_dir, but nothing changed: I always had an index file much smaller than expected.

if is_new_index_file:
    if os.path.isdir(<dirname>):
        rmtree(<dirname>)
        os.mkdir(<dirname>)
    else:
        os.mkdir(<dirname>)
    schema = TranslationSchema()
    index.create_in(<dirname>, <schema>, indexname=<indexname>)
    ix = index.open_dir(<dirname>, indexname=<indexname>, schema=<schema>)
else:
    #open an existing index object
    # ix = index.open_dir(<dirname>, indexname=<indexname>)
    # open file storage
    ix = FileStorage(<dirname>)
    ix.open_index(indexname = <indexname>)
...
list-of-fields = <query-to-the-database-to-extract-fields>
...

writer = ix.writer()
#writer.add_document(<list-of-fields>)
writer.update_document(<list-of-fields>)
writer.commit(merge=False, optimize=True)
ix.close()