Create association of translations with their problems

app/models/problem.rb

@@ -1,3 +1,4 @@
 class Problem < ActiveRecord::Base
   has_one :translation
+  has_many :translations, inverse_of: :problem
 end

app/models/translation.rb

@@ -1,4 +1,6 @@
 class Translation < ActiveRecord::Base
+  belongs_to :problem, inverse_of: :translations
+
   validates :title, :content, presence: true
   validates :title, uniqueness: true