Add find_related_tags method [Austin Foncaier].

CHANGELOG

@@ -2,6 +2,8 @@
 
 * Add TagList#toggle [Pete Yandell].
 
+# Add find_related_tags method [Austin Foncaier].
+
 [30 Jan 08]
 
 * Fix Tag.destroy_unused on Rails 2.0.

lib/acts_as_taggable.rb

@@ -29,7 +29,33 @@ def set_cached_tag_list_column_name(value = nil, &block)
       end
 
       module SingletonMethods
-        # Pass either a tag string, or an array of strings or tags
+        # Returns an array of related tags.
+        # Related tags are all the other tags that are found on the models tagged with the provided tags.
+        # 
+        # Pass either a tag, string, or an array of strings or tags.
+        # 
+        # Options:
+        #   :order - SQL Order how to order the tags. Defaults to "count DESC, tags.name".
+        def find_related_tags(tags, options = {})
+          tags = tags.is_a?(Array) ? TagList.new(tags.map(&:to_s)) : TagList.from(tags)
+
+          related_models = find_tagged_with(tags)
+
+          return [] if related_models.blank?
+
+          related_ids = related_models.to_s(:db)
+
+          Tag.find(:all, options.merge({
+            :select => "#{Tag.table_name}.*, COUNT(#{Tag.table_name}.id) AS count",
+            :joins  => "JOIN #{Tagging.table_name} ON #{Tagging.table_name}.taggable_type = '#{base_class.name}'
+              AND  #{Tagging.table_name}.taggable_id IN (#{related_ids})
+              AND  #{Tagging.table_name}.tag_id = #{Tag.table_name}.id",
+            :order => options[:order] || "count DESC, #{Tag.table_name}.name",
+            :group => "#{Tag.table_name}.id, #{Tag.table_name}.name HAVING #{Tag.table_name}.name NOT IN (#{tags.map { |n| quote_value(n) }.join(",")})"
+          }))
+        end
+
+        # Pass either a tag, string, or an array of strings or tags.
         # 
         # Options:
         #   :exclude - Find models that are not tagged with the given tags

test/acts_as_taggable_test.rb

@@ -2,7 +2,24 @@
 
 class ActsAsTaggableOnSteroidsTest < Test::Unit::TestCase
   fixtures :tags, :taggings, :posts, :users, :photos, :subscriptions, :magazines
+
+  def test_find_related_tags_with
+    assert_equivalent [tags(:good), tags(:bad), tags(:question)], Post.find_related_tags("nature")
+    assert_equivalent [tags(:nature)], Post.find_related_tags([tags(:good)])
+    assert_equivalent [tags(:bad), tags(:question)], Post.find_related_tags(["Very Good", "Nature"])        
+    assert_equivalent [tags(:bad), tags(:question)], Post.find_related_tags([tags(:good), tags(:nature)])
+  end
+
+  def test_find_related_tags_with_non_existent_tags
+    assert_equal [], Post.find_related_tags("ABCDEFG")
+    assert_equal [], Post.find_related_tags(['HIJKLM'])
+  end
 
+  def test_find_related_tags_with_nothing
+    assert_equal [], Post.find_related_tags("")
+    assert_equal [], Post.find_related_tags([])    
+  end
+
   def test_find_tagged_with
     assert_equivalent [posts(:jonathan_sky), posts(:sam_flowers)], Post.find_tagged_with('"Very good"')
     assert_equal Post.find_tagged_with('"Very good"'), Post.find_tagged_with(['Very good'])