ruby on rails - Best way to order across associations -

i have 3 models:

class post   has_many :comments end  class comment   belongs_to :user   belongs_to :post end  class user   has_many :comments end 

now in controller, i'd call @post.comments , order these comments user.postcode. tried following didn't work:

class post   has_many :comments, :order => "user.postcode" end 

i tried:

class comment   def order_by_user_postcode     includes(:user).order("user.postcode asc")   end end  class postscontroller   @post.comments.order_by_user_postcode end 

which results in

undefined method activerecord::relation 

how can write method chain @post.comments sort user.postcode?

you must order:

has_many :comments, :order => "users.postcode" 

table name users not user.

the second option:

you must implement method class method (or scope), not instance method

 def self.order_by_user_postcode    joins(:user).order("users.postcode asc")  end 

or scope:

scope :order_by_user_postcode, joins(:user).order("users.postcode asc")