To generate the current Entity-Relationship Diagram, install graphviz (sudo apt install graphviz or brew install graphviz), then run rake generate_erd. This is also run as a post-migration hook (i.e. after bin/rails db:migrate)
Make sure you have Ruby 2.5.0, Bundler, and PostgreSQL 9.6 installed. Adjust the content of database.yml and .env
cp config/database.yml.example config/database.yml
cp .env.default .env
bundle install
bin/rails db:setup
bin/rails server