Getting "uninitialized constant UserMailer::MailGun" error with Rails + Devise + MailGun integration

Other Questions Mailgun
1

I’m trying to integrate MailGun with Devise authentication in my Rails app using the mailgun-ruby gem. I keep getting an error that says the MailGun constant is not initialized.

In my production.rb file, I added the MailGun configuration:

config.action_mailer.delivery_method = :mailgun
config.action_mailer.mailgun_settings = {
  api_key: ENV['MAILGUN_API_KEY'],
  domain: 'emails.myapp.com'
}

Then I created a custom mailer that extends Devise’s mailer:

class UserMailer < Devise::Mailer
  require 'mailgun-ruby'
  helper :application
  include Devise::Controllers::UrlHelpers
  default template_path: 'users/mailer'

  def confirmation_instructions(user, token, opts={})
    mailgun_client = MailGun::Client.new
    email_params = {
      from: "[email protected]",
      to: user.email,
      subject: "Confirm your email address"
    }

    mailgun_client.send_message email_params

    super
  end
end

I commented out the mailer configuration in devise.rb but Rails seems to think MailGun should be a constant inside UserMailer. What could be causing this issue?

2

you’re overthinking it. set delivery method to mailgun in production.rb and rails does the rest automatically. ditch all that manual mailgun client code from your mailer - just use regular mail(to: user.email, subject: "whatever") syntax. the mailgun config you added is enough.

3

The issue is with the constant name - it should be Mailgun::Client not MailGun::Client. The mailgun-ruby gem uses Mailgun (single word) as the module name, not MailGun with camelCase. I ran into this exact same problem when I first started using the gem. You’re also mixing approaches here. If you’ve already set config.action_mailer.delivery_method = :mailgun in your production.rb, don’t manually instantiate the Mailgun client in your mailer. ActionMailer will handle the delivery through Mailgun automatically. Just change your confirmation_instructions method to use the standard Rails mailer pattern and let ActionMailer do the work. Drop the manual client stuff and use the mail() method like any other Rails mailer.

4

I hit the same issues recently with this exact setup. You’re mixing Rails ActionMailer integration with manual Mailgun client calls, and that’s what’s breaking everything. When you set config.action_mailer.delivery_method = :mailgun in production.rb, Rails handles all the delivery stuff automatically. Your UserMailer should just build the email content - that’s it. Rip out the mailgun_client instantiation and send_message call from your confirmation_instructions method completely. Just use the standard mail() helper method instead. The Mailgun integration catches it automatically when ActionMailer tries to send. Also double-check that your ENV variables are actually loading in production - missing credentials usually show up as constant errors like this.