Final Steps

In the previous guide Building a web application we created a web application using Sequent with Sinatra. In this guide we will continue with that web application and will show you how to add Form validation, and let the Author add Posts.

Adding form validation

Every web application needs some sort of form validation. When creating a Sequent application you typically bind a Command to a web form. A Command represents user intent, like AddPost or AddAuthor. Sequent does not provide any view helpers to render errors in the UI like for instance Rails does. Sequent does however provide a way to do Command validation using the Validation module from Rails. Please check validations in our Reference Guide for all the details. For now we stick to the ‘create author’ form in our web application.

When running the blog application, visiting the home page and directly clicking the ‘Create author’ button (with empty form values), the form blows up with an error:

Sequent::Core::CommandNotValid at /authors
Invalid command AddAuthor 57424bba-1bb3-4cfb-9d64-5b974ff5f3ff, errors: {:name=>["can't be blank"], :email=>["can't be blank"]}

Let’s refresh our minds and see what AddAuthor looks like:

class AddAuthor < Sequent::Command
  attrs name: String, email: String
  validates_presence_of :name, :email
end

Currently we only check whether the name and email attributes are present, but we could add any validates method from the Rails validation module, since it is incorporated into Sequent.{:.notice–info}

In order to provide proper feedback to the user, we need to handle this error in Sinatra and display the error messages at the correct fields.

For this guide we somewhat refactored the web application:

  • Added Bootstrap for a nifty look and feel,
  • Added an erb layout file to manage all displayed erb code (automatically picked up by Sinatra)

Create app/views/layout.erb:

<html>
  <head>
    <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css" integrity="sha384-Gn5384xqQ1aoWXA+058RXPxPg6fy4IWvTNh0E263XmFcJlSAwiGgFAW/dAiS6JXm" crossorigin="anonymous">
  </head>
  <body>
    <div class="container">
      <h1 style="margin-bottom: 20px">Sequent powered Blog</h1>
      <%= yield %>
    </div>
  </body>
</html>

In order to display the error messages in the form we first need to rescue from the Sequent::Core::CommandNotValid in the post '/authors' method.

We will make 2 changes in app/web.rb:

  1. Add a rescue block and store the errors in @errors. We will use this in the erb to display the error messages.
  2. Change the command attribute into an instance variable @command, to allow access from an erb file. This is necessary, since we want to be able to get and display any erroneous values (as housed in the command) the user has entered.

app/web.rb:

  post '/authors' do
    author_id = Sequent.new_uuid
    @command = AddAuthor.from_params(params.merge(aggregate_id: author_id))
    Sequent.command_service.execute_commands @command

    flash[:notice] = 'Account created'
    redirect "/authors/id/#{author_id}"
  rescue Sequent::Core::CommandNotValid => e
    @errors = e.errors
    erb :index
  end

Next we need to change the html form and add some helper methods.

  1. Render form fields in red when they contain an invalid value by changing app/views/index.erb to:
<form method="post" action="/authors">
  <div class="form-group">
    <label for="name">Name</label>
    <input
      id="name"
      name="name"
      type="text"
      value="<%= h @command&.name %>"
      class="form-control <%= error_css_class(:name) %>"
    />
  <div class="form-group">
    <label for="email">Email</label>
    <input
      id="email"
      name="email"
      type="email"
      value="<%= h @command&.email %>"
      class="form-control <%= error_css_class(:email) %>"
    />
  </div>
  <button class="btn btn-primary">Create author</button>
</form>
  1. Add helpers to get the errors pertaining to certain attributes in app/web.rb:
class Web < Sinatra::Base
  ...

  helpers do
    def has_errors_for(attribute)
      @errors && @errors[attribute].present?
    end

    def errors(attribute)
      @errors[attribute] if has_errors_for(attribute)
    end

    def error_css_class(attribute)
      has_errors_for(attribute) ? 'is-invalid' : ''
    end
  end
  
  ...
end

When we now submit an empty form, we can see the input fields are displayed in red.

The last thing we need to do, is display the error messages underneath the relevant input fields.

To enable this functionality, we modify app/views/index.erb:

<form method="post" action="/authors">
  <div class="form-group">
    <label for="name">Name</label>
    <input
      id="name"
      name="name"
      type="text"
      value="<%= h @command&.name %>"
      class="form-control <%= error_css_class(:name) %>"
    />
    <% if has_errors_for(:name) %>
      <div class="invalid-feedback">
        <% errors(:name).each do |error| %>
          <p><%= h error %></p>
        <% end %>
      </div>
    <% end %>
  <div class="form-group">
    <label for="email">Email</label>
    <input
      id="email"
      name="email"
      type="email"
      value="<%= h @command&.email %>"
      class="form-control <%= error_css_class(:email) %>"
    />
    <% if has_errors_for(:email) %>
      <div class="invalid-feedback">
        <% errors(:email).each do |error| %>
          <p><%= h error %></p>
        <% end %>
      </div>
    <% end %>
  </div>
  <button class="btn btn-primary">Create author</button>
</form>

When we now submit an empty form, the error messages are displayed underneath the relevant input fields.

So to summarize, when creating a web application:

  1. In Sequent you typically bind forms to Command objects
  2. You can rescue from the Sequent::Core::CommandNotValid in order to display validation errors from the Commands
  3. Using Sinatra, it is trivial to display those errors the ‘Railsy’ way.

Handling Errors outside Command Validation

What about errors that occur outside Command validation? Remember that we enforce uniqueness of email addresses in the Usernames AggregateRoot. This will raise a Usernames::UsernameAlreadyRegistered error and is not rescued in our web application.

Again this is not something that Sequent handles for you, since it is not a web framework. It is however not that hard to rescue from. Since we only have one custom error class in this example we will rescue this error explicitly.

In app/web.rb:

class Web < Sinatra::Base
  post '/authors' do
    author_id = Sequent.new_uuid
    @command = AddAuthor.from_params(params.merge(aggregate_id: author_id))
    Sequent.command_service.execute_commands @command

    flash[:notice] = 'Account created'
    redirect "/authors/id/#{author_id}"
  rescue Sequent::Core::CommandNotValid => e
    @errors = e.errors
    erb :index
  rescue Usernames::UsernameAlreadyRegistered
    @errors = {email: ['already registered, please choose another']}
    erb :index
  end
end

Tip: If your application grows, it is possible to create a custom base error class for your app and rescue from that in your Sinatra controllers.

Adding and editing Posts

In order to have a fully working blog application, an author needs to be able to submit and edit posts. In this example we won’t go into detail on how to handle logging in, since that is outside of Sequent scope.

For now we will just add the ability to add and edit a Post on the Author’s show page. Since this is somewhat of a repeat of what we did earlier for creating an Author, we just show the code that needs to be added.

Adding the domain logic for editing posts:

In lib/post/commands.rb add:

class EditPost < Sequent::Command
  attrs title: String, content: String
  validates_presence_of :title, :content
end

In lib/post/post_command_handler.rb add:

class PostCommandHandler < Sequent::CommandHandler
  on EditPost do |command|
    do_with_aggregate(command, Post) do |post|
      post.edit(command.title, command.content)
    end
  end
end

In lib/post/post.rb add:

class Post < Sequent::AggregateRoot
  def edit(title, content)
    apply PostTitleChanged, title: title
    apply PostContentChanged, content: content
  end
end

The final version of the PostProjector in app/projectors/post_projector.rb is:

require_relative '../records/post_record'
require_relative '../../lib/post/events'

class PostProjector < Sequent::Projector
  manages_tables PostRecord

  on PostAdded do |event|
    create_record(PostRecord, aggregate_id: event.aggregate_id)
  end

  on PostAuthorChanged do |event|
    update_all_records(
      PostRecord,
      {aggregate_id: event.aggregate_id},
      event.attributes.slice(:author_aggregate_id)
    )
  end

  on PostTitleChanged do |event|
    update_all_records(PostRecord, {aggregate_id: event.aggregate_id}, event.attributes.slice(:title))
  end

  on PostContentChanged do |event|
    update_all_records(PostRecord, {aggregate_id: event.aggregate_id}, event.attributes.slice(:content))
  end
end

In app/web.rb add:

class Web < Sinatra::Base
  post '/authors/id/:author_id/post' do
      post_id = Sequent.new_uuid
  
      @command = AddPost.from_params(
        params.merge(
          aggregate_id: post_id,
          author_aggregate_id: params[:author_id],
        )
      )
      Sequent.command_service.execute_commands @command
  
      flash[:notice] = 'Post created'
  
      redirect "/authors/id/#{params[:author_id]}/post/#{post_id}"
    rescue Sequent::Core::CommandNotValid => e
      @author = AuthorRecord.find_by(aggregate_id: params[:author_id])
      @errors = e.errors
      erb :'authors/show'
    end

  get '/authors/id/:author_id/post/:post_id' do
    @author = AuthorRecord.find_by(aggregate_id: params[:author_id])
    post_record = PostRecord.find_by(aggregate_id: params[:post_id])
    @command = EditPost.new(
      aggregate_id: params[:post_id],
      title: post_record.title,
      content: post_record.content,
    )
    erb :'authors/show'
  end

  post '/authors/id/:author_id/post/:post_id' do
    @command = EditPost.from_params(
      params.merge(
        aggregate_id: params[:post_id],
      )
    )

    Sequent.command_service.execute_commands @command
    flash[:notice] = 'Post saved'
    redirect back
  rescue Sequent::Core::CommandNotValid => e
    @author = AuthorRecord.find_by(aggregate_id: params[:author_id])
    @errors = e.errors
    erb :'authors/show'
  end

  helpers do
    def post_action(command)
      @command&.is_a?(EditPost) ? "/authors/id/#{params[:author_id]}/post/#{command.aggregate_id}" : "/authors/id/#{params[:author_id]}/post"
    end
  end
end

Adding a view for displaying the details of an Author, their posts, and editing/adding a new post:

The complete app/views/authors/show.erb

<div class="container">
  <p>
    <a href="/authors">Back to all authors</a>
  </p>
  <h1>Author <%= h @author.name %> </h1>

  <table class="table">
    <tbody>
      <% @author.post_records.order(:id).each do |post_record| %>
        <tr>
          <td><%= h post_record.title %></td>
          <td>
            <a href="<%= "/authors/id/#{@author.aggregate_id}/post/#{post_record.aggregate_id}" %>">
              <%= h post_record.aggregate_id %>
            </a>
          </td>
        </tr>
      <% end %>
    </tbody>
  </table>

  <p>Email: <%= h @author.email %></p>

  <form method="post" action="<%= post_action(@command) %>">
    <div class="form-group">
      <label for="title">Title</label>
      <input
        id="title"
        name="title"
        type="text"
        value="<%= h @command&.title %>"
        class="form-control <%= error_css_class(:title) %>"
        />
      <% if has_errors_for(:title) %>
        <div class="invalid-feedback">
          <% errors(:title).each do |error| %>
            <p><%= h error %></p>
          <% end %>
        </div>
      <% end %>
    </div>
    <div class="form-group">
      <label for="content">Content</label>
      <textarea
        id="content"
        name="content"
        rows="10"
        class="form-control <%= error_css_class(:content) %>"
      ><%= h @command&.content %></textarea>
      <% if has_errors_for(:content) %>
        <div class="invalid-feedback">
          <% errors(:content).each do |error| %>
            <p><%= h error %></p>
          <% end %>
        </div>
      <% end %>
    </div>

    <button class="btn btn-primary">Save</button>
  </form>
</div>

Since we follow a naming convention in the AddPost command and EditPost command, we can use the same form.

In order to access the post records from our author record, we need to add a has_many relation in app/records/author_record.rb:

  class AuthorRecord < Sequent::ApplicationRecord
    has_many :post_records, foreign_key: 'author_aggregate_id', primary_key: 'aggregate_id'
  end

We need to add this new foreign_key as a new column in the post table.

Update db/tables/post_records.sql:

 CREATE TABLE post_records%SUFFIX% (
     id serial NOT NULL,
     aggregate_id uuid NOT NULL,
     author_aggregate_id uuid,
     title character varying,
     content character varying,
     CONSTRAINT post_records_pkey%SUFFIX% PRIMARY KEY (id)
 );
 
 CREATE UNIQUE INDEX post_records_keys%SUFFIX% ON post_records%SUFFIX% USING btree (aggregate_id);

Lastly update and run the migration as you did in Guide 3. Building a web application > 5. Update and run the migration

Extending Domain Logic

In this guide we have added form validation and added the ability for Authors to add and edit Posts. If your domain requires Authors to keep track of their Posts to enforce a certain business rule, you will explicitly need to add this to your domain logic. This can be done for instance in the PostCommandHandler:

  on AddPost do |command|
    post = Post.new(command)
    repository.add_aggregate(post)
    author = repository.load_aggregate(command.author_aggregate_id, Author)
    author.add_post(post.id)
  end

Of course, the importance of this functionality entirely depends on your domain.

Summary

In this guide we learned about:

  1. Adding the ability for Authors to add and edit Posts
  2. How to add form validation to views through using Sequent Command Validation
  3. Mapping errors to views using rescue Sequent::Core::CommandNotValid