class A < ActiveRecord::Base
  belongs_to :b
end

class B < ActiveRecord::Base
  has_many :as
end

b = B.create
B.transaction do
  b.as.create!
  raise ActiveRecord::Rollback
End
b.save #=> ActiveRecord::StatementInvalid(“Mysql2::Error: Field ‘b_id' doesn't have a default value: INSERT INTO `as` VALUES ()”)