Refactor guardian as dissused in this topic https://meta.discourse.org/t/so-you-want-to-help-out-with-discourse/3823/41?u=hunter

Creates a mixin for the ensure_* functions and creates seperate mixins for functions dealing with posts, categories, and topics.

lib/guardian/ensure_magic.rb

@@ -0,0 +1,22 @@
+# Support for ensure_{blah}! methods.
+module EnsureMagic
+    
+  def method_missing(method, *args, &block)
+    if method.to_s =~ /^ensure_(.*)\!$/
+      can_method = :"#{Regexp.last_match[1]}?"
+
+      if respond_to?(can_method)
+        raise Discourse::InvalidAccess.new("#{can_method} failed") unless send(can_method, *args, &block)
+        return
+      end
+    end
+
+    super.method_missing(method, *args, &block)
+  end
+
+  # Make sure we can see the object. Will raise a NotFound if it's nil
+  def ensure_can_see!(obj)
+    raise Discourse::InvalidAccess.new("Can't see #{obj}") unless can_see?(obj)
+  end
+ 
+end