accepts_nested_attributes_for child association validation failing

Use :inverse_of and validates_presence_of :parent. This should fix your validation problem.

   class Dungeon < ActiveRecord::Base
     has_many :traps, :inverse_of => :dungeon
   end

   class Trap < ActiveRecord::Base
     belongs_to :dungeon, :inverse_of => :traps
     validates_presence_of :dungeon
   end

http://apidock.com/rails/ActiveModel/Validations/HelperMethods/validates_presence_of

https://github.com/rails/rails/blob/73f2d37505025a446bb5314a090f412d0fceb8ca/activerecord/test/cases/nested_attributes_test.rb