Getting Started with Sinatra and Trailblazer

Last updated 09 July 2016

A common misunderstanding is that Trailblazer only works with Rails. This would defeat its philosophy: by decoupling the business logic from the framework, you make it run just anywhere.

This simple example will show you how to write a CRUD interface to create a blog post in a Sinatra environment. We use Sequel as ORM, Trailblazer, and dry-validations for the contract.

The sample application can be found on Github.

Note that this blog post currently only discusses the very simple CRUD aspects of Trailblazer. We will add more chapters as necessary.


Trailblazer makes extensive use of Ruby’s namespaces - a learning from Rails verbose naming madness.

For example, a creating operation for a Post model could be called Post::Create. Likewise, a cell to render the post’s show page could be Post::Cell::Show, an updating contract Post::Contract::Update, and so on.

File Structure

Many frameworks use a file structure by technology: views, models, and so on, are structured in what they do. In Trailblazer, files are grouped by what they are, what concept they represent.

├── app.rb
├── concepts
│   └── post
│       ├── cell
│       │   ├── new.rb
│       │   └── show.rb
│       ├── operation
│       │   ├── create.rb
│       │   └── update.rb
│       └── view
│           ├── new.slim
│           └── show.slim

Files that implement the same group of functions sit in the same directory. This file structure is well-accepted and appreciateed as being more intuitive.


Let’s jump into some very brief code samples now. You will have many questions, most of them we might be able to answer on our Gitter channel. However, you might also consider buying the Trailblazer book. Even though it is written with a Rails example, the generic concepts are well-explained and might help you even better.

Creating Post: Form View

In order to allow users to create a new blog post, we need a form.

I use a cell and a slim view, instead of Sinatra’s built-in rendering stack. Cells is a very popular view model gem from the Rails eco-system, but it is completely decoupled from Rails. It allows ERB, haml and slim templates.

Here’s the view in concepts/post/view/new.slim.

h1 New Post!

form action="#{url}" method="post"
    input type="text" name="title" placeholder="Title"
    input type="text" name="url_slug" placeholder="URL slug"
    textarea name="content" placeholder="And your story..."
    input type="submit"

A very straight-forward form in my favourite template language. Note that this doesn’t use a form builder, yet, and also does not reference any model.

However, we do use a method url in this template that comes from the cell (line 3).

Creating Post: Form Controller

To render this template, we need to invoke the cell in a Sinatra endpoint in app.rb.

get "/posts/new" do
  Post::Cell::New.(nil, url: "/posts").()

I invoke a cell called Post::Cell::New and pass in a :url key.

The cell is implemented in a class and resides in concepts/post/cell/new.rb.

module Post::Cell
  class New < Trailblazer::Cell
    def url
      options[:url] || raise("no action URL!")

Here, I implement the url method that is used in the view and this is how the actual form’s action URL finds its way into the rendered view.

Cells in Trailblazer sit in the concept’s Cell namespace, which is why this particular cells name is Post::Cell::New.

You can now browse to http://localhost:4567/posts/new and you will see the rendered form. Cells are lightning fast and will perform much faster than you might be used from Rails or even from Sinatra.

Creating Post: Processing Controller

When submitting, this form will be POSTed to /posts. We need to add a route for this in app.rb.

post "/posts" do
  op = do |op|
    redirect "/posts/#{}"

  Post::Cell::New.(op, url: "/posts").()

First, I invoke the Post::Create operation that processes the incoming form data. Note how I call run and pass in the params hash. The block is only invoked when the operation was successful and will redirect to the newly created post’s show page.

If not, the remaining block is executed and will re-render the form cell. Note that I pass in the operation instance into the cell.

Creating Post: Operation

The processing operation is implemented in concepts/post/operation/create.rb.

class Post::Create < Trailblazer::Operation
  contract do
    property :title
    property :url_slug
    property :content

  def model!(*)

  def process(params)
    validate(params) do

An operation always has a contract, which is a Reform::Form object. We could easily extract this contract to a separate file concepts/post/contract/create.rb, but for now, it is perfectly fine inline in the operation.

A contract defines fields and validations. We’ll learn about validations soon enough.

The operation can create or find arbitrary models in its model! method. As you can see, this is absolutely not limited to ActiveRecord but can be anything you want.

In the process method, the incoming params then get validated by the form, values get assigned to the contract object, and then, you can do with that whatever you want.

When validating, no data is written to the model. This is all working on the intermediate form twin. Only when you call will data be pushed to the model and saved.

Creating Post: Validation

Let’s add some validations to the contract now. Again, please be aware that in Trailblazer, validations are considered business logic and do not go into the model.

Here’s how the model looks like, in models/post.rb.

class Post < Sequel::Model

Validations go into the contract in concepts/post/operation/create.rb.

class Post::Create < Trailblazer::Operation
  contract do
    property :title
    property :url_slug
    property :content

    validation do
      key(:title, &:filled?)
      key(:url_slug) { |slug| slug.format?(/^[\w-]+$/) && slug.unique? }

      def unique?(value)
        Post[url_slug: value].nil?

Contracts in Trailblazer are Reform objects. Reform allows various validation backends, including the awesome dry-validation, which we use in this application.

I check if the title field is filled out, and if the url_slug has a URL-valid format. Also, I add a simple uniqueness validation. Note how I use Sequel’s API to perform real SQL queries in a validator.

Creating Post: Workflow

When submitting an empty form, we can see the erroring form. It contains the invalid data, but we can’t see errors. Let’s quickly add that to the cell view in concepts/post/view/new.slim.

h1 New Post!

= model.contract.errors.messages.inspect
form action="#{url}" method="post"
  # ..

Since we pass the operation instance to the cell in the POST /posts/ endpoint, we can access this object using the cell’s generic model reader. The operation keeps the contract instance, and so on, enough to display an error message telling the user what went wrong.

Once the form is correctly filled out and submitted, we get redirected to /posts/1.

Viewing Post: Controller

This needs a new endpoint in app.rb.

get "/posts/:id" do
  op = Post::Update.present(params)

  Post::Cell::Show.(op.model, url: "/posts").()