OG Images in Ruby on Rails

Generate OG images in Ruby on Rails with FastOG's subscription-based Dynamic API — with a Dynamic API subscription (create your API key in the dashboard). Preview in the Free OG Image Tester, then render dynamic Open Graph images from your Rails app with HMAC-signed URLs.

Quick Start

1. Create the signer service

ruby
# app/services/fastog_signer.rb
class QuickogSigner
  def self.generate_signed_og_url(params, options = {})
    base_url = options[:base_url] || ENV.fetch("FASTOG_BASE_URL", "https://fastog.com/api/v1/og")
    api_key = options[:api_key] || ENV.fetch("FASTOG_API_KEY")
    hmac_secret = options[:hmac_secret] || ENV.fetch("FASTOG_HMAC_SECRET")

    raise "Missing FASTOG_API_KEY or FASTOG_HMAC_SECRET" unless api_key && hmac_secret

    all = params.merge(key: api_key)
    pairs = all.flat_map do |k, v|
      Array(v).sort.map { |vv| "#{enc(k)}=#{enc(vv)}" }
    end
    canonical = pairs.sort.join("&")
    signature = OpenSSL::HMAC.hexdigest("SHA256", hmac_secret, "GET/api/v1/og\n#{canonical}")
    "#{base_url}?#{URI.encode_www_form(all.merge(s: signature))}"
  end

  def self.enc(s)
    s.to_s.gsub(/[^A-Za-z0-9\-._~]/) { |c| c.bytes.map { |b| "%%%02X" % b }.join }
  end
  private_class_method :enc
end

Why a custom enc? It percent-encodes with uppercase hex, leaving only -._~ unreserved — so spaces become %20, never +, matching the server's rawurlencode. If you swap in CGI.escape or URI.encode_www_form_component, both emit + for spaces: normalize with .gsub('+', '%20') first.

2. Configuration

ruby
# config/application.rb
config.fastog = ActiveSupport::OrderedOptions.new
config.fastog.api_key = ENV.fetch("FASTOG_API_KEY", nil)
config.fastog.hmac_secret = ENV.fetch("FASTOG_HMAC_SECRET", nil)
config.fastog.base_url = ENV.fetch("FASTOG_BASE_URL", "https://fastog.com/api/v1/og")
bash
# .env
FASTOG_API_KEY=sk_your_full_api_key_here
FASTOG_HMAC_SECRET=your_64_char_hex_secret_here

Controller

ruby
# app/controllers/blog_controller.rb
class BlogController < ApplicationController
  def show
    @post = Post.find_by!(slug: params[:slug])

    @og_image_url = QuickogSigner.generate_signed_og_url(
      template: "blog",
      title: @post.title,
      subtitle: @post.excerpt,
      author: @post.author.name
    )
  end
end

View

erb
<!-- app/views/blog/show.html.erb -->
<% content_for :head do %>
  <meta property="og:image" content="<%= @og_image_url %>" />
  <meta property="og:title" content="<%= @post.title %>" />
  <meta property="og:description" content="<%= @post.excerpt %>" />
<% end %>

<article>
  <h1><%= @post.title %></h1>
  <p><%= @post.excerpt %></p>
</article>

Layout

erb
<!-- app/views/layouts/application.html.erb -->
<head>
  <%= yield :head %>
</head>

Helper

ruby
# app/helpers/fastog_helper.rb
module QuickogHelper
  def og_image_url(template:, **params)
    QuickogSigner.generate_signed_og_url({ template: template }.merge(params))
  end
end
erb
<!-- In view -->
<meta property="og:image" content="<%= og_image_url(template: 'blog', title: @post.title) %>" />

E-commerce Example

ruby
# app/controllers/products_controller.rb
class ProductsController < ApplicationController
  def show
    @product = Product.find(params[:id])

    @og_image_url = QuickogSigner.generate_signed_og_url(
      template: "ecommerce",
      title: @product.name,
      price: @product.formatted_price,
      image: @product.image_url
    )
  end
end

Blog Post Example

ruby
# app/controllers/users_controller.rb
class UsersController < ApplicationController
  def show
    @user = User.find_by!(username: params[:username])

    @og_image_url = QuickogSigner.generate_signed_og_url(
      template: "blog",
      title: @user.name,
      subtitle: @user.bio,
      author: @user.name
    )
  end
end

API Endpoint

ruby
# config/routes.rb
get "api/og", to: "api/og#generate"
ruby
# app/controllers/api/og_controller.rb
module Api
  class OgController < ApplicationController
    skip_before_action :verify_authenticity_token

    def generate
      title = params[:title]
      template = params[:template] || "default"

      unless title
        return render json: { error: "Missing title" }, status: :bad_request
      end

      og_image_url = QuickogSigner.generate_signed_og_url(
        template: template,
        title: title
      )

      render json: { ogImageUrl: og_image_url }
    end
  end
end

Turbo/Hotwire

erb
<!-- app/views/blog/_post.html.erb -->
<%= turbo_frame_tag "post_#{@post.id}" do %>
  <meta property="og:image" content="<%= og_image_url(template: 'blog', title: @post.title) %>" />
  <h2><%= link_to @post.title, blog_path(@post) %></h2>
<% end %>

Common Pitfalls

Signing the Wrong Message

❌ Wrong: "GET/api/v1/og#{canonical}" — missing the newline separator. ✅ Correct: "GET/api/v1/og\n#{canonical}" — method + path, a newline, then the canonical query.

Leaving key Out of the Signature

❌ Wrong: signing only your content params and appending key to the URL afterwards. ✅ Correct: key is merged into the params before the canonical query is built, and appears in the final URL.

401 missing_signature / invalid_signature

missing_signature (401) means key or s is missing from the URL. invalid_signature (401) means the canonical didn't match — confirm the newline in the signed message, the same hmac_secret on both sides, and that enc percent-encodes spaces as %20 (never +).

Exposing HMAC Secret

Never expose FASTOG_HMAC_SECRET in frontend JavaScript or client-side code.

Full API Reference

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