SimpleForm + ActiveModel:validation

active_model_account.rb

# app/models/account.rb
class Account
  include ActiveModel::Model
  validates :name, presence: true
  
  def initialize
    # ~ 
  end
end

controller_accounts_controller.rb

# app/controllers/accounts_controller.rb
class AccountsController < ApplicationController
  def new
    @account = Account.new
  end
  
  def create
    @account = Account.new(params[:account])
    if @account.valid?
      @account.save
    else
      render :new
    end
  end
end

views__form.html.haml

# app/views/account/_form.html.haml
## :accountではなく@accountにする
= simple_form_for(@account, url: '/accounts') do |f|
  = f.error_notification

  .form-inputs
    = f.input :name
    
  .form-actions
    = f.button :submit, 'create', class: 'btn-raised'