rails-erd: Generate Entity-Relationship Diagrams for Rails Apps — “Generate a diagram based on your application’s Active Record models. The diagram gives an overview of how your models are related.”
You Don't Need an ER Diagram to Understand Your Database — “ERDs are great for visualizing...
The tool to find inconsistency between models schema and database constraints.
A Rails Engine that generates a page transition diagram for your Rails app from request specs