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

lib/validators/quality_title_validator.rb

@@ -0,0 +1,9 @@
+require 'text_sentinel'
+require 'text_cleaner'
+
+class QualityTitleValidator < ActiveModel::EachValidator
+  def validate_each(record, attribute, value)
+    sentinel = TextSentinel.title_sentinel(value)
+    record.errors.add(attribute, :is_invalid) unless sentinel.valid?
+  end
+end

lib/validators/unique_among_validator.rb

@@ -0,0 +1,21 @@
+class UniqueAmongValidator < ActiveRecord::Validations::UniquenessValidator
+  def validate_each(record, attribute, value)
+    old_errors = record.errors[attribute].size
+
+    # look for any duplicates at all
+    super
+
+    new_errors = record.errors[attribute].size - old_errors
+
+    # do nothing further unless there were some duplicates.
+    unless new_errors == 0
+      # now look only in the collection we care about.
+      dupes = options[:collection].call.where("lower(#{attribute}) = ?", value.downcase)
+      dupes = dupes.where("id != ?", record.id) if record.persisted?
+
+      # pop off the error, if it was a false positive
+      record.errors[attribute].pop(new_errors) unless dupes.exists?
+    end
+  end
+
+end