app/views/mailers/_comments_digest.html.erb
<h3>Comments Summary</h3>
<% comments.each do |comment| %>
<div>Comments on <%= link_to comment.topic.topic, new_topic_comment_url(comment.topic) %></div>
<div><b><%= comment.participant.name %></b></div>
<%= link_to new_topic_comment_url(comment.topic) do %>
<div><%= truncate(strip_tags(comment.comment),length: 50) %>
<% end %>
<br/>
<% end %>
<hr/>