<% @blog.comments.each do |comment| %>
<h3><%= comment.user.email %></h3>
</div>
<% end %>

NoMethodError in Blogs#show
...
undefined method `email' for nil:NilClass

<% @blog.comments.each do |comment| %>
<%= comment.body %>
<% end %>

class Comment < ActiveRecord::Base
  belongs_to :blog
  belongs_to :user
end

<% @users.each do |user| %>
  <tr>
    <td><%= link_to user.username, user %></td>
    <td><%= user.email %></td>
  </tr>
<% end %>

<% @blog.comments.each do |comment| %>
  <h3><%= comment.user.email if comment.user %></h3>
  </div>
<% end %>

<%= comment.user.try(:email) %>

<%= comment.user.try(:email) %>