-svs-: "digiDoc is all about converting paper documents like receipts and business cards into searchable database, and so a document database seemed like a logical fit(!). Alas, not being aware of the mathematics behind relational algebra, I could not see clearly the trap I was falling into - document databases are remarkably hard to run aggregations on and aggregating the data and presenting meaningrful statistics on your receipts is one of the core features of digiDoc. Without the powerful aggregation features that we take for granted in RDBMSs, I would constantly be fighting with unweildy map-reduce constructs when all I want is SUM(amount) FROM receipts WHERE GROUP BY . I even contributed some patches to mongoid-map-reduce but the whole experience of aggregating data with mongodb was so ugh that I couldn’t bring myself to work on the app beyond a point. That is of course, a bad place to be in."
