associations - Show a name from associated class in ruby -

i have aulas , students through grades,

in grades want display name of student , name of aula.

<% @grades.each |grade| %>     <%= grade.student.name %>     <%= grade.aula.name %> <% end %> 

if leave student perfect, when want aula name, get:

undefined method `aula' #<#<class:0x30a37e8>:0x2fffeb0> 

here code

class aula < activerecord::base   attr_accessible :name   has_many :grades   has_many :students, :through => :grades end  class student < activerecord::base   attr_accessible :name   has_many :grades   has_many :aulas, :through => :grades end  class grade < activerecord::base   attr_accessible :grammar, :oral, :participation, :writing   belongs_to :aula   belongs_to :student end 

i figure problem if grade.aula.name nil, error. if data there, works perfectly.

how can action like, if grade.aula.name.nill? grade.aula.name = 'write name here'?