MongoDB doesnt' work after update to macOS Catalina 10.15
Move your old /data/db folder into a non root folder and do the following:

brew uninstall mongodb
brew tap mongodb/brew

brew install mongodb-community


"brew install mongodb" won't work anymore as the mongodb is removed from the homebrew-core. 

check https://github.com/mongodb/homebrew-brew for more info

ref: https://stackoverflow.com/a/57881349/10818622