How to use my view helpers in my ActionMailer views?

In the mailer class that you are using to manage your emails:

class ReportMailer < ActionMailer::Base
  add_template_helper(AnnotationsHelper)

  ...
end

In Rails 3, just use the helper method at the top of your ActionMailer class:

helper :mail   # loads app/helpers/mail_helper.rb & includes MailHelper

I just passed in a block, since I only need it in the one Mailer:

helper do
  def host_url_for(url_path)
    root_url.chop + url_path
  end
end

(be sure to set config.action_mailer.default_url_options.)

(and if you use url_for, be sure to pass in :only_path => false)

For all mailers in Rails 3 (setting "application" helper):

# config/application.rb:
...
config.to_prepare do
  ActionMailer::Base.helper "application"
end

For Ruby on Rails 4, I had to do 2 things:

(1) As Duke already said, if the helper you want to add is UsersHelper for example, then add

helper :users

to the derived ActionMailer class (e.g. app/mailers/user_mailer.rb)

(2) After that, I got a new error:

ActionView::Template::Error (Missing host to link to! Please provide the :host
parameter, set default_url_options[:host], or set :only_path to true)

To fix this, add the line

config.action_mailer.default_url_options = { :host => 'localhost' }

to each of the config/environments/*.rb files. For config/environments/production.rb, replace localhost with a more appropriate host for the production helper-generated urls.

Q: For #2, why does the mail view need this information, and the regular views do not?

A: Because the regular views don't need to know the host, since all generated links are served from the host they link to. Links that show up in emails are not served from the same host (unless you are linking to hotmail.com or gmail.com, etc.)

How to use my view helpers in my ActionMailer views?

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