I definitely recall being on the talk committee one year for DjangoCon, and there was some rough discussion. (Context: The conference was generally a two track conference but keynotes and a few other sessions were single track). One of the single track talks was about Postgres. The discussion was roughly "If we have a Postgres talk we should have another talk like Mongo or MySQL" and the response was roughly "Everyone in Django is using Postgres and if you're not you should be at the talk to learn why you should".

Way more Rails apps used MySQL or other databases, it was largely Heroku winning Rails that led to the strong adoption amongst that community.