If you’ve been using it since 2010, then your usage predates the aggregation framework. I’ll just state for the record that writing a JavaScript mapreduce (or t

If you’ve been using it since 2010, then your usage predates the aggregation framework. I’ll just state for the record that writing a JavaScript mapreduce (or telling a data scientist to write one) to count records is not a good experience. The other issue that got me was I was running queries that should have used an index but indexOnly kept coming back false. I dig into the Mongo JIRA and sure enough it was a bug and queries were returning incorrect values for whether they were using an index. The reason most people got mad at Mongo was probably their benchmark gaming though. They made unsafe writes the default for a long time so their default configuration benchmark would blow away SQL databases which fsync to disk by default (very reasonably!)