tweaks for @bhamman to allow uniqueness validation to be turned off

lib/slug/slug.rb

@@ -8,20 +8,26 @@ module ClassMethods
     #
     # Options:
     # * <tt>:column</tt> - the column the slug will be saved to (defaults to <tt>:slug</tt>)
+    # * <tt>:validates_uniquness_if</tt> - proc to determine whether uniqueness validation runs, same format as validates_uniquness_of :if
     #
-    # Slug will take care of validating presence and uniqueness of slug. It will generate the slug before create;
-    # subsequent changes to the source column will have no effect on the slug.  If you'd like to update the slug
-    # later on, call <tt>@model.set_slug</tt>
+    # Slug will take care of validating presence and uniqueness of slug.
+
+    # Before create, Slug will generate and assign the slug if it wasn't explicitly set.
+    # Note that subsequent changes to the source column will have no effect on the slug.
+    # If you'd like to update the slug later on, call <tt>@model.set_slug</tt>
     def slug source, opts={}
       class_inheritable_accessor :slug_source, :slug_column
       include InstanceMethods
 
       self.slug_source = source
 
       self.slug_column = opts.has_key?(:column) ? opts[:column] : :slug
+
+      uniqueness_opts = {}
+      uniqueness_opts.merge!(:if => opts[:validates_uniqueness_if]) if opts[:validates_uniqueness_if].present?
 
       validates_presence_of     self.slug_column, :message => "cannot be blank. Is #{self.slug_source} sluggable?"
-      validates_uniqueness_of   self.slug_column
+      validates_uniqueness_of   self.slug_column, uniqueness_opts
       validates_format_of       self.slug_column, :with => /^[a-z0-9-]+$/, :message => "contains invalid characters. Only downcase letters, numbers, and '-' are allowed."
       before_validation_on_create :set_slug 
     end
@@ -32,7 +38,7 @@ module InstanceMethods
     # Sets the slug. Called before create.
     def set_slug
       validate_slug_columns
-      self[self.slug_column] = self.send(self.slug_source)
+      self[self.slug_column] = self.send(self.slug_source) if self[self.slug_column].blank?
 
       strip_diacritics_from_slug
       normalize_slug

test/models.rb

@@ -13,6 +13,10 @@ class Company < ActiveRecord::Base
   slug :name
 end
 
+class Post < ActiveRecord::Base
+  slug :headline, :validates_uniqueness_if => Proc.new { false } 
+end
+
 # Used to test slugs based on methods rather than database attributes
 class Event < ActiveRecord::Base
   slug :title_for_slug

test/schema.rb

@@ -16,6 +16,11 @@
     t.column "slug", "string"
   end
 
+  create_table "posts", :force => true do |t|
+    t.column "headline", "string"
+    t.column "slug", "string"
+  end
+
   create_table "events", :force => true do |t|
     t.column "title", "string"
     t.column "location", "string"

test/test_slug.rb

@@ -67,7 +67,30 @@ def setup
     assert !article.valid?
     assert article.errors[:slug].present?
   end
+
+  should "validate uniqueness of slug by default" do
+    article1 = Article.create!(:headline => 'Test Headline')
+    article2 = Article.create!(:headline => 'Test Headline')
+    article2.slug = 'test-headline'
+
+    assert !article2.valid?
+    assert article2.errors[:slug].present?
+  end
 
+  should "use validate_uniquness_if proc to decide whether uniqueness validation applies" do
+    article1 = Post.create!(:headline => 'Test Headline')
+    article2 = Post.new
+    article2.slug = 'test-headline'
+
+    assert article2.valid?
+  end
+
+  should "not overwrite slug value on create if it was already specified" do
+    a = Article.create!(:headline => 'Test Headline', :slug => 'slug1')
+    assert_equal 'slug1', a.slug
+  end
+
+
   context "slug normalization" do
     setup do
       @article = Article.new