Can't create new entity in SQLite with DataMapper

I am new to Ruby world and Sinatra and I am trying to make a database using SQLite3 with Datamapper as ORM. I have installed the gems needed for this action. Here is how I make the db and the model:

DataMapper.setup(:default, "http://sqlite3#{Dir.pwd}/development.db")

class User
  include DataMapper::Resource

  property :id, Serial
  property :email, String, :required => true
  property :first_name, String
  property :last_name, String
  property :password_hash, String
  property :password_salt, String
end

DataMapper.finalize

I have an HTML signup form:

<form action="/signup" method="POST" accept-charset="utf-8">
    <input type="text" name="first_name" placeholder="First Name">
    <input type="text" name="last_name" placeholder="Last Name">
    <input type="mail" name="email" placeholder="Email">
    <input type="password" name="password" placeholder="Password">
    <button type="submit">Register me !</button>
</form>

Here is how i handle the signup action:

post "/signup" do

  password_salt = BCrypt::Engine.generate_salt
  password_hash = BCrypt::Engine.hash_secret(params[:password], password_salt)

  User.create(email: params[:email], first_name: params[:first_name], last_name: params[:last_name], password_hash: password_hash, password_salt: password_salt)

  redirect to '/home'

end

When I execute this and after that I log User.all it does not logs the users I have registered.

But if I run irb and type:

The new user stays in the database.

Any ideas why I can not create users runtime?

P.S. There are no errors logged in the console.