Cross-aggregate projections

When people start doing event sourcing, one mistake that often happens is that developers try to get into the comfort zone of the persisted state of their aggregates as soon as they can. So, many interpret read models as a way to keep the aggregate state accessible in a query able store. I have done the same thing in the past, so you can trust me on that. For the purpose of keeping an aggregate state in some database, I used a nice feature of Event Store—internal projections. You can check the list of available internal projections by visiting the Projections page of the Event Store web UI. One of those projections is $by-category, which links all events to special category streams. For example, $ce-ClassifiedAd will contain all events for the ClassifiedAd aggregate. You can inspect it yourself by visiting http://localhost:2113/web/index.html...