HABTM problem

I have two tables "people" and "relations"
Following are the models
class Person < ActiveRecord::Base
  has_and_belongs_to_many :relations

class Relation < ActiveRecord::Base
  has_and_belongs_to_many :people

I want to select multiple relations and when i say
Person.create(params[:person]) in controller
the relation ids and and id of person should be stored in
people_relation table.

Here is the form for person.
<%form_for (:person,:url=>{:action =>"create"},:html => { :multipart =>
true })do |p|%>
<%= p.collection_select :relation_ids, Relation.find(:all, :order =>
'relation_name ASC'), :id, :relation_name, { :selected => p.relation_ids
}, { :multiple => true, :name => 'p[relation_ids][]' } -%></td>

I am getting error undefined method name relation_ids

how should i write the above piece of code so that the selected
relation_id and person_id go into the people_relations table.

Thank you.