refactor Topic validation

introduce a couple of custom validators
fix minor discrepancies in tests
copy I18n error message keys to default location
clean up validation invocation
move some responsibilities out of validator into class

app/models/topic.rb

@@ -36,15 +36,24 @@ class Topic < ActiveRecord::Base
   rate_limit :limit_topics_per_day
   rate_limit :limit_private_messages_per_day
 
-  validate :title_quality
-  validates_presence_of :title
-  validate :title, -> { SiteSetting.topic_title_length.include? :length }
+  before_validation :sanitize_title
+
+  validates :title, :presence => true,
+                    :length => {  :in => SiteSetting.topic_title_length,
+                                  :allow_blank => true },
+                    :quality_title => { :unless => :private_message? },
+                    :unique_among  => { :unless => Proc.new { |t| (SiteSetting.allow_duplicate_topic_titles? || t.private_message?) },
+                                        :message => :has_already_been_used,
+                                        :allow_blank => true,
+                                        :case_sensitive => false,
+                                        :collection => Proc.new{ Topic.listable_topics } }
+
+  after_validation do
+    self.title = TextCleaner.clean_title(TextSentinel.title_sentinel(title).text) if errors[:title].empty?
+  end
 
   serialize :meta_data, ActiveRecord::Coders::Hstore
 
-  before_validation :sanitize_title
-  validate :unique_title
-
   belongs_to :category
   has_many :posts
   has_many :topic_allowed_users
@@ -142,22 +151,6 @@ class Topic < ActiveRecord::Base
     RateLimiter.new(user, "pms-per-day:#{Date.today.to_s}", SiteSetting.max_private_messages_per_day, 1.day.to_i)
   end
 
-  # Validate unique titles if a site setting is set
-  def unique_title
-    return if SiteSetting.allow_duplicate_topic_titles?
-
-    # Let presence validation catch it if it's blank
-    return if title.blank?
-
-    # Private messages can be called whatever they want
-    return if private_message?
-
-    finder = Topic.listable_topics.where("lower(title) = ?", title.downcase)
-    finder = finder.where("id != ?", self.id) if self.id.present?
-
-    errors.add(:title, I18n.t(:has_already_been_used)) if finder.exists?
-  end
-
   def fancy_title
     return title unless SiteSetting.title_fancy_entities?
 
@@ -168,19 +161,6 @@ class Topic < ActiveRecord::Base
     Redcarpet::Render::SmartyPants.render(title)
   end
 
-  def title_quality
-    # We don't care about quality on private messages
-    return if private_message?
-
-    sentinel = TextSentinel.title_sentinel(title)
-    if sentinel.valid?
-      # clean up the title
-      self.title = TextCleaner.clean_title(sentinel.text)
-    else
-      errors.add(:title, I18n.t(:is_invalid))
-    end
-  end
-
   def sanitize_title
     if self.title.present?
       self.title = sanitize(title, tags: [], attributes: [])