form_for – Ruby on Rails

1. What does the :as => :user say ?

The :as => :user option is used to define the parameter name in the form submission. For example:

<%= form_for @user, :as => :user, :url => sign_in_path(@user) do |f| %>

When the form is submitted, Rails will group the submitted fields under a key named user. For instance:

Parameters: { "user" => { "email" => "test@example.com", "password" => "password" } }
If you leave out :as, Rails will infer the name based on the model (e.g., @user defaults to user).

you could use :as => :custom_key if you wanted the submitted parameters to be nested under params[:custom_key] instead of params[:user]

2. :url => sign_in_path is clear, but why is there a (@user) behind it?

sign_in_path is a Rails route helper, generating a URL for a specific action.
@user is passed as an argument to include information about the @user object in the URL.

#routes.rb
get 'sign_in/:id', to: 'sessions#new', as: 'sign_in'

In this case, sign_in_path(@user) generates a URL like /sign_in/123, assuming @user.id is 123.

3. how to access @user object in application.html.erb

Solution: Use a before_action in ApplicationController
The ApplicationController is a base class for all controllers in your Rails application, so adding a before_action here will make @user available across all views, including layouts.

class ApplicationController < ActionController::Base
  before_action :set_user
  private
  def set_user
    # Assuming current_user is a method that returns the logged-in user
    @user = current_user
  end
end

4. <%= form_for (User.new) works well, but it isn't right.

It’s not ideal for Clarity and Readability, Controller Responsibility, Consistency
A Better Approach:
Initialize the model object in the controller and use the instance variable in the view.
Controller:-

class UsersController < ApplicationController
  def new
    @user = User.new
  end
end

View:-

<%= form_for @user do |f| %>
  
<% end %>

Benefits of This Approach:

Encapsulation : The controller is responsible for preparing the data, and the view focuses on rendering it.

Flexibility: If you need to modify how @user is initialized (e.g., with default attributes), you only update the controller.

Testing: Makes your views easier to test because the required data is explicit.

5. Why is there something like a for/forEach-loop? do |f| %>

form_for: This helper is used to create a form for a specific model object. It binds the form to the model and allows Rails to manage form fields and parameters efficiently.
Block: The do ... end block (or { ... } in shorter form) is passed to the form_for helper, and inside the block, you define the fields and structure of the form.
-> f: The form builder object provided by form_for. You use this object to generate fields like text_field, password_field, etc.

# Syntax
<%= form_for @user do |f| %>
    <%= f.label :name %>
    <%= f.text_field :name %>
   <%= f.label :email %>
    <%= f.email_field :email %>
    <%= f.submit "Save" %>
<% end %>