search: reduce queries in single tag searches (#4120).

Avoid some queries used in wiki page excerpts:

* Only try to load the artist if the tag is an artist tag.
* Avoid using `exists?` queries for wiki pages.
* Bugfix: don't show wiki excerpts for deleted wikis.

app/logical/post_sets/base.rb

@@ -1,9 +1,5 @@
 module PostSets
   class Base
-    def has_wiki?
-      false
-    end
-
     def raw
       false
     end
@@ -12,10 +8,6 @@ module PostSets
       nil
     end
 
-    def has_artist?
-      false
-    end
-
     def artist
       nil
     end

app/logical/post_sets/post.rb

@@ -25,24 +25,14 @@ module PostSets
       tag_array.reject {|tag| tag =~ /\Aorder:/i }
     end
 
-    def has_wiki?
-      is_single_tag? && ::WikiPage.titled(tag_string).exists? && wiki_page.visible?
-    end
-
-    def has_wiki_text?
-      has_wiki? && wiki_page.body.present?
-    end
-
     def has_blank_wiki?
-      is_simple_tag? && !has_wiki?
+      tag.present? && !wiki_page.present?
     end
 
     def wiki_page
-      if is_single_tag?
-        ::WikiPage.titled(tag_string).first
-      else
-        nil
-      end
+      return nil unless tag.present? && tag.wiki_page.present?
+      return nil unless !tag.wiki_page.is_deleted? && tag.wiki_page.visible?
+      tag.wiki_page
     end
 
     def tag
@@ -50,12 +40,10 @@ module PostSets
       @tag ||= Tag.find_by(name: Tag.normalize_name(tag_string))
     end
 
-    def has_artist?
-      is_single_tag? && artist.present? && artist.visible?
-    end
-
     def artist
-      @artist ||= ::Artist.named(tag_string).active.first
+      return nil unless tag.present? && tag.category == Tag.categories.artist
+      return nil unless tag.artist.present? && tag.artist.is_active? && tag.artist.visible?
+      tag.artist
     end
 
     def pool_name

app/views/posts/index.html.erb

@@ -26,9 +26,9 @@
       <menu id="post-sections">
         <li class="active"><a href="#" id="show-posts-link">Posts</a></li>
 
-        <% if @post_set.has_artist? %>
+        <% if @post_set.artist.present? %>
           <li><%= link_to "Artist", artist_path(@post_set.artist), :id => "show-excerpt-link" %></li>
-        <% elsif @post_set.has_wiki? %>
+        <% elsif @post_set.wiki_page.present? %>
           <li><%= link_to "Wiki", wiki_page_path(@post_set.wiki_page), :id => "show-excerpt-link" %></li>
         <% elsif @post_set.has_pool? %>
           <li><%= link_to "Pool", pool_path(@post_set.pool), :id => "show-excerpt-link" %></li>

app/views/posts/partials/index/_excerpt.html.erb

@@ -1,7 +1,7 @@
 <%# post_set %>
 
 <div id="excerpt" style="display: none;">
-  <% if post_set.has_artist? %>
+  <% if post_set.artist.present? %>
     <% post_set.artist.tap do |artist| %>
       <% if artist.visible? %>
         <% unless artist.notes.blank? %>
@@ -21,7 +21,7 @@
         </div>
       <% end %>
     <% end %>
-  <% elsif post_set.has_wiki? %>
+  <% elsif post_set.wiki_page.present? %>
     <% post_set.wiki_page.tap do |wiki_page| %>
       <div class="prose">
         <% if wiki_page.other_names.present? %>