diff --git a/CHANGELOG.rdoc b/CHANGELOG.rdoc
index c9ed045..c4078b2 100644
--- a/CHANGELOG.rdoc
+++ b/CHANGELOG.rdoc
@@ -1,3 +1,5 @@
+* Don't set resource instance variable if it has been set already - see issue #13
+
 * Allowing :nested option to accept an array for deep nesting
 
 * Adding :nested option to load resource method - see issue #10
diff --git a/lib/cancan/controller_additions.rb b/lib/cancan/controller_additions.rb
index 85684bc..0a7b5a7 100644
--- a/lib/cancan/controller_additions.rb
+++ b/lib/cancan/controller_additions.rb
@@ -4,8 +4,8 @@ module CanCan
   # It also makes the "can?" and "cannot?" methods available to all views.
   module ControllerAdditions
     module ClassMethods
-      # Sets up a before filter which loads and authorizes the current resource. This accepts the
-      # same arguments as load_resource and authorize_resource. See those methods for details.
+      # Sets up a before filter which loads and authorizes the current resource. This performs both
+      # load_resource and authorize_resource and accepts the same arguments. See those methods for details.
       # 
       #   class BooksController < ApplicationController
       #     load_and_authorize_resource
@@ -21,12 +21,26 @@ module CanCan
       # Article.new(params[:article]) depending upon the action. It does nothing for the "index"
       # action.
       # 
-      # You would call this method directly on the controller class.
+      # Call this method directly on the controller class.
       # 
       #   class BooksController < ApplicationController
       #     load_resource
       #   end
       # 
+      # A resource is not loaded if the instance variable is already set. This makes it easy to override
+      # the behavior through a before_filter on certain actions.
+      # 
+      #   class BooksController < ApplicationController
+      #     before_filter :find_book_by_permalink, :only => :show
+      #     load_resource
+      #
+      #     private
+      #
+      #     def find_book_by_permalink
+      #       @book = Book.find_by_permalink!(params[:id)
+      #     end
+      #   end
+      # 
       # See load_and_authorize_resource to automatically authorize the resource too.
       # 
       # Options:
@@ -70,7 +84,7 @@ module CanCan
       # 
       #   unauthorized! if cannot?(params[:action].to_sym, @article || Article)
       # 
-      # You would call this method directly on the controller class.
+      # Call this method directly on the controller class.
       # 
       #   class BooksController < ApplicationController
       #     authorize_resource
diff --git a/lib/cancan/controller_resource.rb b/lib/cancan/controller_resource.rb
index 677acd0..d057f35 100644
--- a/lib/cancan/controller_resource.rb
+++ b/lib/cancan/controller_resource.rb
@@ -11,14 +11,14 @@ module CanCan
     end
     
     def find(id)
-      self.model_instance = base.find(id)
+      self.model_instance ||= base.find(id)
     end
     
     def build(attributes)
       if base.kind_of? Class
-        self.model_instance = base.new(attributes)
+        self.model_instance ||= base.new(attributes)
       else
-        self.model_instance = base.build(attributes)
+        self.model_instance ||= base.build(attributes)
       end
     end
     
diff --git a/spec/cancan/controller_resource_spec.rb b/spec/cancan/controller_resource_spec.rb
index 9c29d8c..40e77d0 100644
--- a/spec/cancan/controller_resource_spec.rb
+++ b/spec/cancan/controller_resource_spec.rb
@@ -34,4 +34,10 @@ describe CanCan::ControllerResource do
     CanCan::ControllerResource.new(@controller, :ability, parent).build(123)
     @controller.instance_variable_get(:@ability).should == :some_ability
   end
+  
+  it "should not load resource if instance variable is already provided" do
+    @controller.instance_variable_set(:@ability, :some_ability)
+    CanCan::ControllerResource.new(@controller, :ability).find(123)
+    @controller.instance_variable_get(:@ability).should == :some_ability
+  end
 end