Wrap host lookup so it's easily stubbable

app/models/user.rb

@@ -83,11 +83,15 @@ 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)
 
     template_id = BETA_ONBOARDING_TEMPLATE_ID
-    url = edit_user_password_url({ host: ENV["APP_HOST"] })
+    url = edit_user_password_url({ host: })
     token = Devise.token_generator.generate(User, :reset_password_token)
     personalisation = { name: name || email, link: "#{url}?reset_password_token=#{token}" }
     DeviseNotifyMailer.new.send_email(email, template_id, personalisation)

spec/lib/tasks/onboarding_emails_spec.rb

@@ -17,7 +17,7 @@ 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(ENV).to receive(:[]).with("APP_HOST").and_return("http://localhost:3000")
+    allow(user).to receive(:host).and_return("http://localhost:3000")
   end
 
   context "when onboarding a new organisation to private beta" do