Rails - Seeking a Dry authorization method compatible with various nested resources

Question

Consensus is you shouldn't nest resources deeper than 1 level. So if I have 3 models like this (below is just a hypothetical situation)

User has_many Houses has_many Tenants

and to abide by the above i do

map.resources :users, :has_many => :houses
map.resorces :houses, :has_many => :tenants

Now I want the user to be able edit both their houses and their tenants details but I want to prevent them from trying to edit another users houses and tenants by forging the user_id part of the urls. So I create a before_filter like this

 def prevent_user_acting_as_other_user
        if User.find_by_id(params[:user_id]) != current_user()
            @current_user_session.destroy
            flash[:error] = "Stop screwing around wiseguy"
            redirect_to login_url()
            return
        end
    end

for houses that's easy because the user_id is passed via

edit_user_house_path(@user, @house)

but in the tenents case

tenant house_tenent_path(@house)

no user id is passed. But I can get the user id by doing @house.user.id but then id have to change the code above to this.

    def prevent_user_acting_as_other_user
    if params[:user_id]
        @user = User.find(params[:user_id]
    elsif params[:house_id]
        @user = House.find(params[:house_id]).user
    end
    if @user != current_user()
        #kick em out
    end
end

It does the job, but I'm wondering if there is a more elegant way. Every time I add a new resource that needs protecting from user forgery Ill have to keep adding conditionals. I don't think there will be many cases but would like to know a better approach if one exists.

Answer 1

Do the following:

class User < ActiveRecord::Base
  has_many :houses
  has_many :tenants
end

class House < ActiveRecord::Base
  belongs_to :user
  has_many :tenants
end

class Tenant < ActiveRecord::Base
  belongs_to :user
  belongs_to :house
end

In your filter do the following:

def kill_session(message)
  @current_user_session.destroy
  flash[:error] = message
   redirect_to login_url()
end

def prevent_user_acting_as_other_user
  if    params[:user_id]  and params[:user_id] != @current_user.id
    kill_session("You don't have access to this page")
  elsif params[:house_id] and !@current_user.houses.exists?(params[:house_id])
    kill_session("You don't have access to this page")
  elsif params[:tenant_id] and !@current_user.tenants.exists?(params[:tanant_id])
    kill_session("You don't have access to this page")
  end
end