OpenApiAnnotator Gem Version Build Status Maintainability Test Coverage

OpenApiAnnotator realizes to generate OpenAPI spec by annotating to controllers and serializers. If you use ActiveModelSerializer, this is the best way to generate OpenAPI spec.


Add this line to your application's Gemfile:

gem 'open_api_annotator'


Annotating controllers and serializers, you can generate OpenAPI spec file from these. Things you have to do are three below:

  1. Configure API meta information
  2. Annotate controllers
  3. Annotate serializers

1. Configure API meta information

You have to set API meta information like:

# config/initializers/open_api_annotator.rb
OpenApiAnnotator.configure do |config| = "Book API", version: "1")
  config.destination_path = Rails.root.join("api_spec.yml")
  config.path_regexp = /\Aapi\/v1\// # If you want to restrict a path to create

2. Annotate controller

To define an entity of an endpoint, call the method endpoint in the previous line of an action method. It takes entity expression as the first arg. Entity expression is a model class or an array that contains only one model class.

class Api::V1::BooksController
  endpoint [Book] # 👈It means an array of Book
  def index
    books = Book.limit(10)
    render json: books

  endpoint Book # 👈Just a Book
  def show
    book = Book.find(params[:id])
    render json: book

  endpoint Book # 👈Just a Book
  def update
     book = Book.find(params[:id])
     render json: book

3. Annotate serializer

To define an schema in components, set type, format, nullable as each field option.

class BookSerializer < ApplicationSerializer
  attribute :title, type: :string, nullable: false
  attribute :published_at, type: :string, format: :"date-time", nullable: true

  has_many :authors, type: [Author], nullable: false
  has_one :cover_image, type: CoverImage, nullable: true
  belongs_to :publisher, type: Publisher, nullable: false


After checking out the repo, run bin/setup to install dependencies. Then, run rake spec to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to


Bug reports and pull requests are welcome on GitHub at This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the Contributor Covenant code of conduct.


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

Code of Conduct

Everyone interacting in the OpenApiAnnotator project’s codebases, issue trackers, chat rooms and mailing lists is expected to follow the code of conduct.