Set slug to previous value on validation failure

On create, set slug to nil if unrelated validations fail.

On save, set slug to previous value if unrelated validations fail.

Without this change, Rails forms will sometimes use a new, unpersisted
slug value as the form action URL, and submitting the form will result
in a 404.

Resolves #642

lib/friendly_id/slugged.rb

@@ -86,7 +86,7 @@ def self.down
     class Person < ActiveRecord::Base
       extend FriendlyId
       friendly_id :name_and_location, use: :slugged
-      
+
       def name_and_location
         "#{name} from #{location}"
       end
@@ -248,6 +248,7 @@ def self.included(model_class)
         defaults[:sequence_separator] ||= '-'
       end
       model_class.before_validation :set_slug
+      model_class.after_validation :unset_slug_if_invalid
     end
 
     # Process the given value to make it suitable for use as a slug.
@@ -324,6 +325,14 @@ def slug_generator
     end
     private :slug_generator
 
+    def unset_slug_if_invalid
+      if errors.present? && attribute_changed?(friendly_id_config.query_field)
+        diff = changes[friendly_id_config.query_field]
+        self.slug = diff.first
+      end
+    end
+    private :unset_slug_if_invalid
+
     # This module adds the `:slug_column`, and `:sequence_separator`, and
     # `:slug_generator_class` configuration options to
     # {FriendlyId::Configuration FriendlyId::Configuration}.

test/slugged_test.rb

@@ -92,6 +92,47 @@ def should_generate_new_friendly_id?
     end
   end
 
+  test "should not set slug on create if unrelated validations fail" do
+    klass = Class.new model_class do
+      validates_presence_of :active
+      friendly_id :name, :use => :slugged
+
+      def self.name
+        "Journalist"
+      end
+    end
+
+    transaction do
+      instance = klass.new :name => 'foo'
+      refute instance.save
+      refute instance.valid?
+      assert_nil instance.slug
+    end
+  end
+
+  test "should not update slug on save if unrelated validations fail" do
+    klass = Class.new model_class do
+      validates_presence_of :active
+      friendly_id :name, :use => :slugged
+
+      def self.name
+        "Journalist"
+      end
+    end
+
+    transaction do
+      instance = klass.new :name => 'foo', :active => true
+      assert instance.save
+      assert instance.valid?
+      instance.name = 'foobar'
+      instance.slug = nil
+      instance.active = nil
+      refute instance.save
+      refute instance.valid?
+      assert_equal 'foo', instance.slug
+    end
+  end
+
 end
 
 class SlugGeneratorTest < TestCaseClass