Send emails via sidekiq worker

app/mailers/emails/admin_notification.rb

+module Emails
+  module AdminNotification
+    def send_admin_notification(user_id, subject, body)
+      @body = body
+      mail to: recipient(user_id), subject: subject
+    end
+  end
+end
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app/mailers/notify.rb

 class Notify < ActionMailer::Base
   include ActionDispatch::Routing::PolymorphicRoutes
 
+  include Emails::AdminNotification
   include Emails::Issues
   include Emails::MergeRequests
   include Emails::Notes

app/views/notify/send_admin_notification.html.haml

+= simple_format @body
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app/views/notify/send_admin_notification.text.haml

+= h @body
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app/workers/admin_emails_worker.rb

+class AdminEmailsWorker
+  include Sidekiq::Worker
+
+  def perform(recipient_id, subject, body)
+    recipient_list(recipient_id).pluck(:id).each do |user_id|
+      Notify.send_admin_notification(user_id, subject, body).deliver
+    end
+  end
+
+  private
+  def recipient_list(recipient_id)
+    case recipient_id
+    when 'all'
+      User.where(nil)
+    when /group-(\d+)\z/
+      Group.find($1).users
+    when /project-(\d+)\z/
+      Project.find($1).users
+    end
+  end
+end
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