Change target_id to null on todos

db/migrate/20160316192622_change_target_id_to_null_on_todos.rb

@@ -0,0 +1,5 @@
+class ChangeTargetIdToNullOnTodos < ActiveRecord::Migration
+  def change
+    change_column_null :todos, :target_id, true
+  end
+end

db/schema.rb

@@ -867,7 +867,7 @@ ActiveRecord::Schema.define(version: 20160316204731) do
   create_table "todos", force: :cascade do |t|
     t.integer  "user_id",     null: false
     t.integer  "project_id",  null: false
-    t.integer  "target_id",   null: false
+    t.integer  "target_id"
     t.string   "target_type", null: false
     t.integer  "author_id"
     t.integer  "action",      null: false