using link_to inside if - ruby-on-rails

Question

I have a user_authenticate method which is setting the #current_user based on session. I have a navbar layout which contains the navigation links. I want to hide one of the links and show it only if user id is 8 (which is admin). I have written the layout like this.
<div align="center" style="width: 90%">
<nav class="navbar navbar-inverse" role="navigation">
<%= link_to "Home", '/', :class => "btn btn-default navbar-btn"%>
<%= link_to "Currency Master", currencies_url, :class => "btn btn-default navbar-btn"%>
<%= link_to "Receipts", receipts_url, :class => "btn btn-default navbar-btn"%>
<%= link_to "Expenses", expenses_url, :class => "btn btn-default navbar-btn"%>
<%= link_to "Exchange", exchanges_url, :class => "btn btn-default navbar-btn"%>
<% if #current_user.id == 8 %><%= link_to "Report", reports_url, :class => "btn btn-default navbar-btn" %>
<% end %>
</nav>
The code works fine that it shows the "Report"button on navbar only of the user id is 8. But as soon as I click on the link I get following error.
undefined method `id' for nil:NilClass
Extracted source (around line #8):
6
7
8
9
10
<%= link_to "Expenses", expenses_url, :class => "btn btn-default navbar-btn"%>
<%= link_to "Exchange", exchanges_url, :class => "btn btn-default navbar-btn"%>
<% if #current_user.id == 8 %><%= link_to "Report", reports_url, :class => "btn btn-default navbar-btn" %>
<% end %>
</nav>

Answer 1

First you should not check if the user is an Admin by ID. You can have an "is_admin" column of boolean type and check like this:
if #current_user.is_admin?
# ...
Second, before check the value of a column, you can check first if the object itself exist. So your if condition can be like this:
if #current_user && #curent_user.is_admin?
# show admin menu
else
# show other menus
end

Answer 2

You are checking #current_user.id even if #current_user is not present. so it is giving an error.
First check the presence of current_user and then check id,
#current_user.present? && #current_user.id == 8
Change this line,
<% if #current_user.id == 8 %><%= link_to "Report", reports_url, :class => "btn btn-default navbar-btn" %>
<% end %>
to,
<% if (#current_user.present? && #current_user.id == 8) %><%= link_to "Report", reports_url, :class => "btn btn-default navbar-btn" %>
<% end %>