fixes #2415: "Random post" causes RecordNotFound exceptions

app/controllers/posts_controller.rb

@@ -93,7 +93,7 @@ class PostsController < ApplicationController
 
   def random
     count = Post.fast_count(params[:tags], :statement_timeout => CurrentUser.user.statement_timeout)
-    @post = Post.tag_match(params[:tags]).reorder("").offset(rand(count)).first
+    @post = Post.tag_match(params[:tags]).random
     raise ActiveRecord::RecordNotFound if @post.nil?
     respond_with(@post) do |format|
       format.html { redirect_to post_path(@post, :tags => params[:tags]) }

app/logical/post_sets/post.rb

@@ -91,29 +91,9 @@ module PostSets
     end
 
     def get_random_posts
-      if unknown_post_count?
+      per_page.times.inject([]) do |all, x|
-        chance = 0.01
+        all << ::Post.tag_match(tag_string).random
-      elsif post_count == 0
+      end.compact.uniq
-        chance = 1
-      else
-        chance = per_page / post_count.to_f
-      end
-
-      temp = []
-      temp += ::Post.tag_match(tag_string).where("random() < ?", chance).reorder("").limit(per_page)
-
-      3.times do
-        missing = per_page - temp.length
-        if missing >= 1
-          q = ::Post.tag_match(tag_string).where("random() < ?", chance*2).reorder("").limit(missing)
-          unless temp.empty?
-            q = q.where("id not in (?)", temp.map(&:id))
-          end
-          temp += q
-        end
-      end
-
-      temp
     end
 
     def posts

app/models/post.rb

@@ -1579,6 +1579,20 @@ class Post < ActiveRecord::Base
   end
 
   module SearchMethods
+    # returns one single post
+    def random
+      key = Digest::MD5.hexdigest(Time.now.to_f.to_s)
+      random_up(key) || random_down(key)
+    end
+
+    def random_up(key)
+      where("md5 < ?", key).reorder("md5 desc").first
+    end
+
+    def random_down(key)
+      where("md5 >= ?", key).reorder("md5 asc").first
+    end
+
     def pending
       where("is_pending = ?", true)
     end

db/structure.sql

@@ -7072,6 +7072,13 @@ CREATE INDEX index_users_on_last_ip_addr ON users USING btree (last_ip_addr) WHE
 CREATE UNIQUE INDEX index_users_on_name ON users USING btree (lower((name)::text));
 
 
+--
+-- Name: index_users_on_name_trgm; Type: INDEX; Schema: public; Owner: -
+--
+
+CREATE INDEX index_users_on_name_trgm ON users USING gin (lower((name)::text) gin_trgm_ops);
+
+
 --
 -- Name: index_wiki_page_versions_on_updater_ip_addr; Type: INDEX; Schema: public; Owner: -
 --
@@ -7441,3 +7448,5 @@ INSERT INTO schema_migrations (version) VALUES ('20170112021922');
 
 INSERT INTO schema_migrations (version) VALUES ('20170112060921');
 
+INSERT INTO schema_migrations (version) VALUES ('20170117233040');
+