notification when merge request reassigned

app/mailers/notify.rb

@@ -45,4 +45,12 @@ class Notify < ActionMailer::Base
     @project = merge_request.project
     mail(:to => @user.email, :subject => "gitlab | #{@merge_request.title} ")
   end
+  
+  def changed_merge_request_email(user, merge_request)
+    @user = user
+    @assignee_was ||= User.find(merge_request.assignee_id_was)
+    @merge_request = merge_request
+    @project = merge_request.project
+    mail(:to => @user.email, :subject => "gitlab | #{@merge_request.title} ")
+  end
 end

app/models/mailer_observer.rb

@@ -9,6 +9,10 @@ class MailerObserver < ActiveRecord::Observer
     new_merge_request(model) if model.kind_of?(MergeRequest)
   end
 
+  def after_update(model)
+    changed_merge_request(model) if model.kind_of?(MergeRequest)
+  end
+
   protected
 
     def new_issue(issue)
@@ -45,4 +49,16 @@ class MailerObserver < ActiveRecord::Observer
       end
     end
 
+    def changed_merge_request(merge_request)
+      if merge_request.assignee_id_changed?
+        recipients_ids = [merge_request.assignee_id_was]
+        recipients_ids << merge_request.assignee_id
+        recipients_ids.delete current_user.id
+
+        User.find(recipients_ids).each do |user|
+          Notify.changed_merge_request_email(user, merge_request).deliver
+        end
+      end
+    end
+
 end

app/views/notify/changed_merge_request_email.html.haml

+%td.content{:align => "left", :style => "font-family: Helvetica, Arial, sans-serif; padding: 20px 0 0;", :valign => "top", :width => "600"}
+  %table{:border => "0", :cellpadding => "0", :cellspacing => "0", :style => "color: #717171; font: normal 11px Helvetica, Arial, sans-serif; margin: 0; padding: 0;", :width => "600"}
+    %tr
+      %td{:style => "font-size: 1px; line-height: 1px;", :width => "21"}
+      %td{:align => "left", :style => "padding: 20px 0 0;"}
+        %h2{:style => "color:#646464; font-weight: bold; margin: 0; padding: 0; line-height: 26px; font-size: 18px; font-family: Helvetica, Arial, sans-serif; "}
+          Reassigned Merge Request
+          = link_to truncate(@merge_request.title, :length => 16), project_merge_request_url(@project, @merge_request)
+      %td{:style => "font-size: 1px; line-height: 1px;", :width => "21"}
+    %tr
+      %td{:style => "font-size: 1px; line-height: 1px;", :width => "21"}
+      %td{:style => "padding: 15px 0 15px;", :valign => "top"}
+        %p{:style => "color:#767676; font-weight: normal; margin: 0; padding: 0; line-height: 20px; font-size: 12px;font-family: Helvetica, Arial, sans-serif; "}
+          Assignee changed from #{@assignee_was.name} to #{@merge_request.assignee.name}
+      %td
+