schemerd
Auto-generate Mermaid ER diagrams from your ActiveRecord models. Keeps your schema documentation in sync by regenerating after every migration.
Installation
Add to your Gemfile:
group :development do
gem "schemerd"
endRun bundle install.
Usage
Generate the diagram
rake schemerd:generateThis creates docs/erd.md with a Mermaid erDiagram block containing all your models, columns, and associations.
Auto-generate on migrations
By default, Schemerd hooks into db:migrate, db:migrate:up, db:migrate:down, and db:migrate:redo in development. The diagram regenerates automatically after each migration.
To disable this:
Schemerd.configure do |config|
config.auto_generate = false
endConfiguration
Generate an initializer with all available options:
rails generate schemerd:installThis creates config/initializers/schemerd.rb. Available options:
Schemerd.configure do |config|
# Output directory relative to Rails.root (default: "docs")
config.output_directory = "docs"
# Output filename (default: "erd.md")
config.output_filename = "erd.md"
# Header text at the top of the generated file
config.header = "# Entity Relationship Diagram\n\nAuto-generated. Do not edit manually."
# Model name prefixes to exclude from the diagram (default: [])
config.excluded_prefixes = ["Flipper::", "Ahoy::"]
# Automatically regenerate after migrations (default: true)
config.auto_generate = true
# Base class for model discovery (default: "ApplicationRecord")
config.base_class = "ApplicationRecord"
endOutput Example
The generated file contains a fenced Mermaid code block:
# Entity Relationship Diagram
Auto-generated from ActiveRecord models. Do not edit manually.
```mermaid
erDiagram
Post }o--|| User : "author"
Comment ||--o{ Post : "comments"
User {
integer id PK
string name
string email
datetime created_at
datetime updated_at
}
Post {
integer id PK
string title
text body
integer user_id
datetime created_at
datetime updated_at
}
```Render it on GitHub, GitLab, or any Mermaid-compatible viewer.
License
MIT License. See LICENSE.txt.