Schema ERD

Your Rails schema, alive.

Schema ERD turns db/schema.rb into an interactive database map inside your Rails app. Mount one Rack endpoint and get a canvas you can search, pan, zoom, rearrange, and explore—without Graphviz, Node, model loading, or a build step.

# config/routes.rb
mount SchemaErd::App.new(schema_path: Rails.root.join("db/schema.rb")), at: "/rails/erd"

Open /rails/erd. That is the whole setup.

Why Schema ERD?

Most ERD tools generate an artifact. Schema ERD gives you a workspace.

• Reads the source of truth. Tables, columns, indexes, defaults, null constraints, primary keys, and foreign keys come straight from db/schema.rb.
• Feels immediate. Refresh the page after a migration; there is nothing to regenerate.
• Handles real schemas. Search tables and columns, hide Rails internals, toggle detail, and fit the entire graph to the window.
• Makes relationships navigable. Click a table to isolate its references and dependents. The selected table lives in the URL, ready to paste into a pull request or team chat.
• Gets out of your way. Drag cards into place, pan the canvas, zoom with the controls or keyboard, and let local storage remember the layout.
• Ships almost nothing. The rendered page is standalone HTML, CSS, and JavaScript. The gem has no runtime dependencies.

Installation

Schema ERD is a development tool. Keep it out of production:

# Gemfile
group :development do
  gem "schema_erd"
end

Then mount it behind a development-only route:

# config/routes.rb
Rails.application.routes.draw do
  mount SchemaErd::App.new(schema_path: Rails.root.join("db/schema.rb")), at: "/rails/erd" if Rails.env.development?
end

Run bundle install, boot Rails, and visit http://localhost:3000/rails/erd.

Controls

Action	Control
Find a table or column	Search box
Focus a table and its neighbors	Click its header
Move a table	Drag its header
Pan	Drag empty canvas space
Zoom	Buttons, + / -, or Ctrl/⌘ + wheel
Reset zoom	0
Fit the diagram	F
Leave focused view	Esc or “Show all tables”

Layout positions are stored in the browser and keyed by the schema version. A new migration gets a fresh layout; older layouts stay intact.

Rails internals

Schema ERD recognizes common framework-owned tables such as Active Storage, sessions, ar_internal_metadata, and schema_migrations. They are visible by default and can be hidden from the controls.

Requirements and scope

• Ruby 3.2 or newer.
• A Rails-style Ruby schema file (db/schema.rb).
• Any Rack-compatible Rails version capable of mounting an object that responds to call.

structure.sql is not supported. Schema ERD parses the stable shape of Rails-generated Ruby schema files; it does not execute the schema or boot your models.

Use outside Rails

The app is an ordinary Rack endpoint, so Rails is convenient rather than required:

require "schema_erd"

run SchemaErd::App.new(schema_path: File.expand_path("db/schema.rb", __dir__))

The parser is public too:

schema = SchemaErd::Parser.call(File.read("db/schema.rb"))

schema.tables.first.name
schema.relationships.first.from_table

Security

An ERD exposes table and column names. Mount it only where intended—normally in development—and add your own authentication if you expose it in a shared environment. Responses are marked no-store and noindex, nofollow, but those headers are not access control.

Development

bundle install
bundle exec rake
bundle exec rake build

Bug reports and focused pull requests are welcome. Please include a minimal schema.rb fragment when reporting parser behavior.

License

Schema ERD is available under the MIT License.