My data-parsing-spreadsheet-tool-turned-web-app needed to accept arbitrarily-large documents... I realized after the fact that cookies only stored so much data.

So it was time to learn databases.

I took a time-out to learn and practice SQL, and used that knowledge to build a few relationshipped SQLAlchemy models on the app's backend, which connected it to a PostgreSQL db deployed alongside the app on Heroku.