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
 

db/migrate/20141214221259_add_problem_to_translation.rb

@@ -0,0 +1,5 @@
+class AddProblemToTranslation < ActiveRecord::Migration
+  def change
+    add_reference :translations, :problem, index: true
+  end
+end

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20141214220056) do
+ActiveRecord::Schema.define(version: 20141214221259) do
 
   create_table "problems", force: true do |t|
     t.datetime "created_at"
@@ -26,6 +26,9 @@ ActiveRecord::Schema.define(version: 20141214220056) do
     t.text     "content"
     t.datetime "created_at"
     t.datetime "updated_at"
+    t.integer  "problem_id"
   end
 
+  add_index "translations", ["problem_id"], name: "index_translations_on_problem_id"
+
 end