added ability to not regenerate slugs on save, using :assign_slug? me…

…thod on Mongoid object. See specs.

lib/mongoid/slugify.rb

@@ -9,40 +9,9 @@ module Slugify
     included do
       field :slug
       index :slug, :unique => true
-      before_save :assign_slug
+      before_save :assign_slug, :if => :assign_slug?
     end
 
-    def to_param
-      slug || super
-    end
-
-    private
-      def generate_slug
-        raise NotImplementedError
-      end
-
-      def generate_unique_slug
-        current_slug = generate_slug
-        pattern = /^#{Regexp.escape(current_slug)}(?:-(\d+))?$/
-
-        appropriate_class = self.class
-        while (appropriate_class.superclass.include?(Mongoid::Document))
-          appropriate_class = appropriate_class.superclass
-        end
-
-        existing_slugs = appropriate_class.where(:slug => pattern, :_id.ne => _id).only(:slug).map { |record| record.slug }
-        if existing_slugs.count > 0
-          max_counter = existing_slugs.map { |slug| (pattern.match(slug)[1] || 0).to_i }.max
-          current_slug << "-#{max_counter + 1}"
-        end
-
-        current_slug
-      end
-
-      def assign_slug
-        self.slug = generate_unique_slug
-      end
-
     module ClassMethods
       def find_by_slug(slug)
         where(:slug => slug).first
@@ -60,5 +29,40 @@ def find_by_slug_or_id!(slug_or_id)
         find_by_slug(slug_or_id) || find(slug_or_id)
       end
     end
+
+    def to_param
+      slug || super
+    end
+
+    private
+    def assign_slug?
+      true
+    end
+
+    def generate_slug
+      raise NotImplementedError
+    end
+
+    def generate_unique_slug
+      current_slug = generate_slug
+      pattern = /^#{Regexp.escape(current_slug)}(?:-(\d+))?$/
+
+      appropriate_class = self.class
+      while (appropriate_class.superclass.include?(Mongoid::Document))
+        appropriate_class = appropriate_class.superclass
+      end
+
+      existing_slugs = appropriate_class.where(:slug => pattern, :_id.ne => _id).only(:slug).map { |record| record.slug }
+      if existing_slugs.count > 0
+        max_counter = existing_slugs.map { |slug| (pattern.match(slug)[1] || 0).to_i }.max
+        current_slug << "-#{max_counter + 1}"
+      end
+
+      current_slug
+    end
+
+    def assign_slug
+      self.slug = generate_unique_slug
+    end
   end
 end

spec/mongoid/slugify_spec.rb

@@ -32,6 +32,13 @@ def generate_slug
 class ComicBook < Book
 end
 
+class CookBook < Book
+  private
+  def assign_slug?
+    slug.blank?
+  end
+end
+
 class Person
   include Mongoid::Document
   include Mongoid::Slugify
@@ -98,6 +105,36 @@ module Mongoid
       end
     end
 
+    context "when assign_slug? is configured to work only when slug is empty" do
+      let(:book) do
+        CookBook.create(:title => "A Thousand Plateaus")
+      end
+
+      it "generates a slug" do
+        book.to_param.should eql "a-thousand-plateaus"
+      end
+
+      it "does not update the slug" do
+        book.title = "Anti Oedipus"
+        book.save
+        book.to_param.should eql "a-thousand-plateaus"
+      end
+
+      it "updates the slug if it's nil" do
+        book.title = "Anti Oedipus"
+        book.slug = nil
+        book.save
+        book.to_param.should eql "anti-oedipus"
+      end
+
+      it "updates the slug if it's empty" do
+        book.title = "Anti Oedipus"
+        book.slug = ""
+        book.save
+        book.to_param.should eql "anti-oedipus"
+      end
+    end
+
     context "when the slug is composed of multiple fields" do
       let!(:author) do
         Author.create(