rails - what exactly does hidden_field and hidden_field_tag do?

Question

I read through the techy definition of hidden_fields, but am not sure what it really does. My understanding is that it allows you to pass in an attribute for c

Answer 1

It would generate a hidden type of input field...

<input type="hidden" />

This is a way to store information that you want submitted with the form without having to have a visible field.

From the documentation:

hidden_field_tag 'tags_list' generates...
<input id="tags_list" name="tags_list" type="hidden" />

hidden_field_tag is meant to be used without a model whereas hidden_field is meant to be used in conjunction with a form_for call and a model.

hidden_field(:signup, :pass_confirm) generates...
<input type="hidden" id="signup_pass_confirm" name="signup[pass_confirm]" value="#{@signup.pass_confirm}" />

Answer 2

Both of those methods are helpers to create an HTML input tag of type "hidden", and yes, those are used to add parameters to a request (typically a form POST). Really the parameter can be any piece of information you want to send along with a request. Be careful, though, as hidden fields are easily tampered with.

Here's an example that will send a user id in a hidden field

# Form
<%= form_tag foo_path do %>
  <%= hidden_field_tag "user_id", @user.id %>
  ....
  <%= submit_tag "Click Me" %>
<% end %>

# Controller
def foo
  # params[:user_id] is set with the value from the hidden field
  # Do useful stuff with the POST data
end

While you can pass things such as user_id's like this, I find that the need for it is rare. If a user_id is always required for a given situation you might consider using nested routes http://guides.rubyonrails.org/routing.html#nested-resources.