Rake task can be called outside the rails environment so need to pass host in

app/models/user.rb

@@ -83,12 +83,8 @@ class User < ApplicationRecord
     DeviseNotifyMailer.new.send_email(email, template_id, personalisation)
   end
 
-  def host
-    ENV["APP_HOST"]
-  end
-
-  def send_beta_onboarding_email
-    return unless URI::MailTo::EMAIL_REGEXP.match?(email)
+  def send_beta_onboarding_email(host)
+    return unless URI::MailTo::EMAIL_REGEXP.match?(email) && host
 
     template_id = BETA_ONBOARDING_TEMPLATE_ID
     url = edit_user_password_url({ host: })

lib/tasks/onboarding_emails.rake

@@ -2,13 +2,15 @@ include Rails.application.routes.url_helpers
 
 namespace :onboarding_emails do
   desc "Send onboarding emails to private beta users"
-  task :send, %i[organisation_id] => :environment do |_task, args|
+  task :send, %i[organisation_id host] => :environment do |_task, args|
     organisation_id = args[:organisation_id]
+    host = args[:host]
     raise "Organisation id must be provided" unless organisation_id
+    raise "host must be provided" unless host
 
     organisation = Organisation.find(organisation_id)
     raise "Organisation #{organisation_id} does not exist" unless organisation
 
-    organisation.users.each(&:send_beta_onboarding_email)
+    organisation.users.each { |user| user.send_beta_onboarding_email(host) }
   end
 end

spec/lib/tasks/onboarding_emails_spec.rb

@@ -4,6 +4,7 @@ require "rake"
 describe "rake onboarding_emails:send", type: task do
   subject(:task) { Rake::Task["onboarding_emails:send"] }
 
+  context "when onboarding a new organisation to private beta" do
     let!(:user) { FactoryBot.create(:user) }
     let(:notify_client) { instance_double(Notifications::Client) }
     let(:devise_notify_mailer) { DeviseNotifyMailer.new }
@@ -17,10 +18,8 @@ describe "rake onboarding_emails:send", type: task do
       allow(devise_notify_mailer).to receive(:notify_client).and_return(notify_client)
       allow(notify_client).to receive(:send_email).and_return(true)
       allow(Devise.token_generator).to receive(:generate).and_return(reset_password_token)
-    allow(user).to receive(:host).and_return("http://localhost:3000")
     end
 
-  context "when onboarding a new organisation to private beta" do
     it "can send the onboarding emails" do
       expect(notify_client).to receive(:send_email).with(
         {
@@ -33,7 +32,7 @@ describe "rake onboarding_emails:send", type: task do
         },
       )
 
-      task.invoke(user.organisation.id)
+      task.invoke(user.organisation.id, "http://localhost:3000")
     end
   end
 end