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Rails Mermaid ERD

Generate Mermaid ERD from your Ruby on Rails application.

Demo Page

Mermaid ERD can be generated at will. The generated ERD can be copied in Markdown format, so they can be easily shared on GitHub. You can also save it as an image, so it can be used in environments where Mermaid is not available. The editor is a single HTML file, so the entire editor can be shared.

Installation

Add this line to your application's Gemfile:

gem "rails-mermaid_erd", group: :development, require: false

And then execute:

$ bundle install

Add this line to your application's Rakefile:

begin
  require "rails-mermaid_erd"
rescue LoadError
  # Do nothing.
end

Usage

Run rake task mermaid_erd will generate <app_root>/mermaid_erd/index.html.

$ bundle exec rails mermaid_erd
# or
$ bundle exec rake mermaid_erd

Simply open the generated <app_root>/mermaid_erd/index.html in your browser.

This file is not required for Git management, so you can add it to .gitignore if necessary

mermaid_erd

<app_root>/mermaid_erd/index.html is a single self-contained HTML file. All front-end dependencies (Tailwind, Mermaid, Vue) are inlined, so it works offline and behind strict corporate proxies — no CDN access is needed at view time. The file is roughly 4 MB because the bundles ship inside it.

If you share this file, it can be used by those who do not have a Ruby on Rails environment. Or, you can upload the file to a web server and share it with the same URL.

It would be very smart to generate it automatically using CI.

Print the Mermaid source to stdout

Run rake task mermaid_erd:print to print the raw erDiagram source to stdout instead of writing the HTML viewer. This pipes cleanly into other tools:

$ bundle exec rails mermaid_erd:print
$ bundle exec rails mermaid_erd:print > er.mmd
$ bundle exec rails mermaid_erd:print | mmdc -i - -o er.svg

The output is the full diagram — every table, column, key, comment, and relation — equivalent to the HTML viewer with all of its detail toggles enabled.

Languages

The viewer UI ships in 12 languages: English, 日本語, 简体中文, 繁體中文, 한국어, Español, Français, Deutsch, Italiano, Português (Brasil), Русский, and العربية (right-to-left). It auto-detects the browser language (navigator.language) on load, falls back to English for unsupported locales, and can be switched manually from the selector in the top-right corner.

Supported versions

The Ruby × Rails combinations exercised by CI on every push and pull request:

Rails	Ruby
5.2	2.7
6.0	2.7, 3.0
6.1	2.7, 3.0, 3.1, 3.2
7.0	2.7, 3.0, 3.1, 3.2, 3.3
7.1	3.1, 3.2, 3.3, 3.4
7.2	3.1, 3.2, 3.3, 3.4
8.0	3.2, 3.3, 3.4, 4.0
8.1	3.2, 3.3, 3.4, 4.0

Other combinations may work but are not verified.

Configuration

./config/mermaid_erd.yml to customize the configuration. See ./docs/example.yml for an example configuration.

The setting items are as follows.

key	description	default
result_path	Destination of generated files.	mermaid_erd/index.html
ignore_tables	Array of regular-expression strings. Tables whose table_name matches any pattern are dropped from the generated ERD, along with any relations that point at them. Useful for excluding audit-log models, soft-deleted/legacy tables, or other large noise you don't want to render. Patterns are compiled with Regexp.new, so escape backslashes inside YAML strings (e.g. "\\Aaudit_").	[]
viewer_defaults	The view the viewer opens on when the URL carries no saved state, and the one Reset returns to; a link with its own state always wins. models: array of model names selected at first (a name that is not in the diagram is skipped with a warning when the file is generated). columns: all, keys (primary and foreign keys only) or none, the same choices as Columns in the sidebar. Useful for opening a large app on the models people usually start from.	models: [] (nothing selected), columns: all

License

The gem is available as open source under the terms of the MIT License.