app/views/topic_mailer/added_to_map.text.erb
<% topic = @entity || entity %>
<% event = @event || event %>
<%= event.user.name %> added topic <%= topic.name %> to map <%= event.map.name %>
topic_url(topic)
map_url(event.map)
<%= render :partial => 'topic_mailer/unfollow', locals: { topic: topic } %>