Passing dynamic subject to Rails mail_form - ruby-on-rails

Question

I am using gem mail_form to handle contact in a Rails app, and I have 6 different contact forms.
I put a hidden_field_tag in the form and pass the desired subject as a variable. In the html, the value is there but the email arrives with (no subject). What can I be doing wrong?
In controllers
def where_to_buy
#contact = Contact.new
#the_subject = "Where to buy"
end
In contact form
= form_for #contact do |f|
= render "form", f: f
= f.text_area :message
.hide
= f.text_field :nickname, hint: 'Leave this field empty!'
= hidden_field_tag :mail_subject, #the_subject
= f.submit "Send Message"
In model
class Contact < MailForm::Base
attribute :mail_subject
attribute :first_name, validate: true
attribute :last_name, validate: true
attribute :message, validate: true
attribute :nickname, captcha: true
def headers
{
subject: %(#{mail_subject}),
to: "jorge#email123.com",
from: %("#{first_name} #{last_name}" <#{email}>)
}
end
end
Output html in chrome:
<input type="hidden" name="mail_subject" id="mail_subject" value="Where to buy">

Answer 1

Instead of:
= hidden_field_tag :mail_subject, #the_subject
You will want to use:
= f.hidden_field :mail_subject, value: #the_subject
If you inspect the parameters that are logged into your development.log you will see why.
When you use hidden_field_tag the mail_subject is defined as its own independent parameter and will not be included in the contact hash. You'll have something like:
params = { "contact" => { "message" => "text here", ... }, "mail_subject => "Where to buy" }
But when you use f.hidden_field the mail_subject will be included in the contact hash. You'll have something like:
params = { "contact" => { "message" => "text here", "mail_subject => "Where to buy", ... } }
And then when you call Contact.new(params[:contact]) the new contact object will get the mail_subject value.