Check out example codes for "DEPRECATION WARNING: render file: should be given the absolute path to a file". It will help you in understanding the concepts better.

Code Example 1

diff --git a/app/views/layouts/admin.html.erb b/app/views/layouts/admin.html.erb
index 8f23129b4..0ce74d104 100644
--- a/app/views/layouts/admin.html.erb
+++ b/app/views/layouts/admin.html.erb
@@ -5,4 +5,4 @@
   <% end %>
 <% end %>

-<%= render :file => "layouts/base" %>
+<%= render :template => 'layouts/base' %>
</diff>

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