ruby on rails - accepts_nested_attributes_for :reject_if to trigger another method -

i've got multi-level nested form using formtastic_cocoon (jquery version of formtastic).

i trying validation in sense of

 if value is_numeric      insert database else      database lookup on text      insert id association end 

i hoping tha accepts_nested_attributes_for have :if option, apparently there :reject_if.

is there way create validation describe part of accepts_nested_attributes_for??

-----------------------updated per zubin's response ---------------------------

i believe zubin on right track method, can't seem working right. method using is

      def lookup_prereq=(lookup_prereq)         return if lookup_prereq.blank?             case lookup_prereq         when lookup_prereq.is_a?(numeric) == true             self.task_id = lookup_prereq         else             self.task = task.find_by_title(lookup_prereq)         end     end  

when trigger function, self.task_id being put in database '0' rather task.id.

i'm wondering if i'm missing else. i'm not sure method being called. shouldn't need

lookup_prereq(attr[:prereq_id) 

at point?

-------------------further edit ----------------------- think can find method called if named same name value database, therefore i've changed method

 def completed_task=(completed_task) 

unfortunately still resulting in 0 value in database.

sounds need method in nested model handle that, eg:

class post < activerecord::base   has_many :comments   accepts_nested_attributes_for :comments end  class comment < activerecord::base   belongs_to :post   belongs_to :author    def lookup_author=(lookup_author)     return if lookup_author.blank?     case lookup_author     when /^\d+$/       self.author_id = lookup_author     else       self.author = author.find_by_name(lookup_author)     end   end end