You would like to have an ERD diagram of your database in your Ruby on Rails project.
You can install the Rails ERD from here, and install it by following the instructions.
For Ubuntu/debian systems should be:
sudo apt-get install graphviz
and then adding the gem to your Gemfile in the development section as:
group :development do gem "rails-erd", "~> 0.4.5" end
You can then run bundle install to install the gem and rake erb to create the diagram pdf.