Finishing the web application with Sequent
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
:
- Add a rescue block and store the errors in
@errors
. We will use this in the erb to display the error messages. - 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.
- 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>
- 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:
- In Sequent you typically bind forms to
Command
objects - You can rescue from the
Sequent::Core::CommandNotValid
in order to display validation errors from the Commands - 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 Author
s to add and edit Post
s.
If your domain requires Author
s to keep track of their Post
s 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:
- Adding the ability for
Author
s to add and editPost
s - How to add form validation to views through using Sequent Command Validation
- Mapping errors to views using
rescue Sequent::Core::CommandNotValid