gistfile1.rb

class User < ActiveRecord::Base
  def find_matches
    User.where(:city => self.city)
    # self == the user object you called this function on
  end
end

class UsersController < ApplicationController
  def match
    @user = User.find(params[:id])
    @matches = @user.find_matches
    # @matches is an array of user objects (look at users#index to see how to iterate over an array of user objects
  end
end