Fix #14: Make uniqueness check among other problems

app/models/translation.rb

@@ -2,7 +2,16 @@ class Translation < ActiveRecord::Base
   belongs_to :problem, inverse_of: :translations
 
   validates :title, :content, :problem_id, presence: true
-  validates :title, uniqueness: true
+  validate :title_is_unique_among_other_problems
 
   self.per_page = 50
+
+  def title_is_unique_among_other_problems
+    Problem.where.not(id: problem_id).each do |problem|
+      if problem.is_translated? and problem.title == title
+        errors.add(:title, :taken)
+        break
+      end
+    end
+  end
 end