FEATURE: add a way to map arbitrary urls to a topic, post, or category. Useful for sites that have migrated to Discourse and want to redirect from their old site to Discourse with 301 redirects.

app/controllers/permalinks_controller.rb

@@ -0,0 +1,12 @@
+class PermalinksController < ApplicationController
+  skip_before_filter :check_xhr, :preload_json
+
+  def show
+    permalink = Permalink.find_by_url(params[:url])
+    if permalink && permalink.target_url
+      redirect_to permalink.target_url, status: :moved_permanently
+    else
+      raise Discourse::NotFound
+    end
+  end
+end

app/models/permalink.rb

@@ -0,0 +1,21 @@
+class Permalink < ActiveRecord::Base
+  belongs_to :topic
+  belongs_to :post
+  belongs_to :category
+
+  before_validation :normalize_url
+
+  def normalize_url
+    if self.url
+      self.url = self.url.strip
+      self.url = self.url[1..-1] if url[0,1] == '/'
+    end
+  end
+
+  def target_url
+    return post.url if post
+    return topic.relative_url if topic
+    return category.url if category
+    nil
+  end
+end

config/application.rb

@@ -148,6 +148,10 @@ module Discourse
     require 'auth'
     Discourse.activate_plugins! unless Rails.env.test? and ENV['LOAD_PLUGINS'] != "1"
 
+    initializer :add_last_routes, :after => :add_routing_paths do |app|
+      app.routes_reloader.paths << File.join(Rails.root, 'config', 'routes_last.rb')
+    end
+
     config.after_initialize do
       # So open id logs somewhere sane
       OpenID::Util.logger = Rails.logger

config/routes.rb

@@ -425,4 +425,5 @@ Discourse::Application.routes.draw do
   # special case for top
   root to: "list#top", constraints: HomePageConstraint.new("top"), :as => "top_lists"
 
+  # See config/routes_last.rb for more
 end

config/routes_last.rb

@@ -0,0 +1,10 @@
+# These routes must be loaded after all others.
+# Routes are loaded in this order:
+#
+#  1. config/routes.rb
+#  2. routes in engines
+#  3. config/routes_last.rb
+
+Discourse::Application.routes.draw do
+  get "*url", to: 'permalinks#show'
+end

db/migrate/20140828172407_create_permalinks.rb

@@ -0,0 +1,14 @@
+class CreatePermalinks < ActiveRecord::Migration
+  def change
+    create_table :permalinks do |t|
+      t.string :url, null: false
+      t.integer :topic_id
+      t.integer :post_id
+      t.integer :category_id
+
+      t.timestamps
+    end
+
+    add_index :permalinks, :url
+  end
+end

spec/controllers/permalinks_controller_spec.rb

@@ -0,0 +1,19 @@
+require 'spec_helper'
+
+describe PermalinksController do
+  describe 'show' do
+    it "should redirect to a permalink's target_url with status 301" do
+      permalink = Fabricate(:permalink)
+      Permalink.any_instance.stubs(:target_url).returns('/t/the-topic-slug/42')
+      get :show, url: permalink.url
+      response.should redirect_to('/t/the-topic-slug/42')
+      response.status.should == 301
+    end
+
+    it 'return 404 if permalink record does not exist' do
+      get :show, url: '/not/a/valid/url'
+      response.status.should == 404
+    end
+  end
+
+end

spec/fabricators/permalink_fabricator.rb

@@ -0,0 +1,3 @@
+Fabricator(:permalink) do
+  url { sequence(:url) {|i| "my/#{i}/url" } }
+end

spec/models/permalink_spec.rb

@@ -0,0 +1,72 @@
+require "spec_helper"
+
+describe Permalink do
+
+  describe "new record" do
+    it "strips blanks" do
+      permalink = described_class.create(url: " my/old/url  ")
+      permalink.url.should == "my/old/url"
+    end
+
+    it "removes leading slash" do
+      permalink = described_class.create(url: "/my/old/url")
+      permalink.url.should == "my/old/url"
+    end
+  end
+
+  describe "target_url" do
+
+    let(:permalink)       { Fabricate.build(:permalink) }
+    let(:topic)           { Fabricate(:topic) }
+    let(:post)            { Fabricate(:post, topic: topic) }
+    let(:category)        { Fabricate(:category) }
+    subject(:target_url)  { permalink.target_url }
+
+    it "returns a topic url when topic_id is set" do
+      permalink.topic_id = topic.id
+      target_url.should == topic.relative_url
+    end
+
+    it "returns nil when topic_id is set but topic is not found" do
+      permalink.topic_id = 99999
+      target_url.should be_nil
+    end
+
+    it "returns a post url when post_id is set" do
+      permalink.post_id = post.id
+      target_url.should == post.url
+    end
+
+    it "returns nil when post_id is set but post is not found" do
+      permalink.post_id = 99999
+      target_url.should be_nil
+    end
+
+    it "returns a post url when post_id and topic_id are both set" do
+      permalink.post_id = post.id
+      permalink.topic_id = topic.id
+      target_url.should == post.url
+    end
+
+    it "returns a category url when category_id is set" do
+      permalink.category_id = category.id
+      target_url.should == category.url
+    end
+
+    it "returns nil when category_id is set but category is not found" do
+      permalink.category_id = 99999
+      target_url.should be_nil
+    end
+
+    it "returns a post url when topic_id, post_id, and category_id are all set for some reason" do
+      permalink.post_id = post.id
+      permalink.topic_id = topic.id
+      permalink.category_id = category.id
+      target_url.should == post.url
+    end
+
+    it "returns nil when nothing is set" do
+      target_url.should be_nil
+    end
+  end
+end