Auto-generate OpenAPI schema field properties from ActiveModel validations

rswag_spec_helper.rb

# Configure for rswag:specs:swaggerize

RSpec.configure do |config|
  config.openapi_specs = {
    "v1/swagger.yaml" => {
      components: {
        schemas: {
          user: User.object_for_rswag(:id, :first_name, :last_name, :email, :portal_role, :status)
        }
      }
    }
  }
end

swagger.yaml

components:
  schemas:
    user:
      type: object
      required:
      - id
      - first_name
      - last_name
      - email
      - status
      properties:
        id:
          type: string
          format: uuid
        first_name:
          type: string
          minLength: 1
          maxLength: 70
        last_name:
          type: string
          minLength: 1
          maxLength: 70
        email:
          type: string
          maxLength: 250
          pattern: "\\A[a-zA-Z0-9.!\\#$%&'*+/=?^_`{|}~-]+@[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?(?:\\.[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)*\\z"

swagger_schema_helper.rb

module SwaggerSchemaHelper
  def attribute_for_rswag(attribute)
    validators = validators_on(attribute).reject { it.is_a?(ActiveRecord::Validations::UniquenessValidator) }
    return { type: :string, format: :uuid } if validators.empty?

    validators.each_with_object({}) do |validator, options|
      case validator
      when ActiveRecord::Validations::PresenceValidator
        next
      when ActiveModel::Validations::LengthValidator
        options[:type] = :string
        options[:minLength] = validator.options[:minimum] if validator.options[:minimum]
        options[:maxLength] = validator.options[:maximum] if validator.options[:maximum]
      when ActiveModel::Validations::InclusionValidator
        options[:type] = :string
        options[:enum] = validator.options[:in]
      when ActiveModel::Validations::FormatValidator
        options[:pattern] = validator.options[:with].source
      when ActiveRecord::Validations::NumericalityValidator
        range = validator.options[:in]
        options[:type] = validator.options[:only_integer] ? :integer : :number
        options[:minimum] = range.begin if range&.begin
        options[:maximum] = range.end if range&.end
      else
        raise "unhandled #{validator.class.name}"
      end

      options[:nullable] = true if validator.options[:allow_nil]
    end
  end

  def object_for_rswag(*attributes)
    properties = attributes.to_h { [ it, attribute_for_rswag(it) ] }

    { type: :object, required: properties.keys.select { !properties[it].key?(:nullable) }, properties: }
  end
end

ApplicationRecord.singleton_class.include SwaggerSchemaHelper

user.rb

# app/models/user.rb

class User < ApplicationRecord
  validates :first_name, presence: true, length: { in: 1..70 }
  validates :last_name, presence: true, length: { in: 1..70 }
  validates :email, presence: true, uniqueness: true, length: { maximum: 250 },
                    format: { with: URI::MailTo::EMAIL_REGEXP }
end