activerecord - Undefined method error when using "has_many :through" relationship on Rails3 -

i working on small android project ror server. here 3 models:

class user < activerecord::base       has_many :relations       has_many :friends, :through => :relations       attr_accessor :friend_ids end  class relation < activerecord::base    belongs_to :user    belongs_to :friend end  class friend < activerecord::base    has_many :relations    has_many :users, :through => :relations end   class createusers < activerecord::migration  def self.up create_table :users |t|   t.string :user_name   t.string :password   t.integer :city_id   t.integer :travelstyle_id   t.boolean :online   t.string :self_description   t.string :sex   t.integer :head_id    t.timestamps end 

end

def self.down   drop_table :users end 

end

 class createfriends < activerecord::migration  def self.up create_table :friends |t|   t.string :user_name   t.integer :city_id   t.integer :travelstyle_id   t.string :self_description   t.string :sex   t.integer :head_id   t.timestamps end 

end

  class createrelations < activerecord::migration def self.up create_table :relations |t|   t.integer :user_id   t.integer :friend_id   t.timestamps end 

end

model user uses model relation connect model friend. use scaffold creat 3 models , add relationship code in model files. create api controller send xml file android application. here controller code:

def find_friend   @user=user.where("id=?",params[:id])   @friend=@user.friends respond_to |format|   format.html   format.xml end 

end

here problem, when use api(type in http://localhost:3000/api/find_friend/1.xml), server throws mistake:

nomethoderror in apicontroller#find_friend  undefined method `friends' #<activerecord::relation:0x3478a28> app/controllers/api_controller.rb:21:in `find_friend' 

i new rails , have no idea wrong is. have add in "route.rb" or change migration file?

in rails console mode, type in "user=user.find(1), friend=user.friends" , correct result.

~~~~(>_<)~~~~ problem controller method "@user=user.where("id=?",params[:id])". "where" method can not tell whether result array or 1 object. if use "@user=user.find(params[:id])", rails "smart enough" know "oh, yes, 1 object , has method called friends because connects 2 models together". learning rails likes marriage, think know think "god know nothing mysterious guy."