fixes #1891

app/logical/related_tag_calculator.rb

@@ -9,6 +9,29 @@ class RelatedTagCalculator
     convert_hash_to_array(calculate_from_sample(tags, Danbooru.config.post_sample_size, category_constraint))
   end
 
+  def self.calculate_from_post_set_to_array(post_set, category_constraint = nil)
+    convert_hash_to_array(calculate_from_post_set(post_set, category_constraint))
+  end
+
+  def self.calculate_from_post_set(post_set, category_constraint = nil)
+    counts = Hash.new {|h, k| h[k] = 0}
+
+    post_set.posts.each do |post|
+      post.tag_array.each do |tag|
+        category = Tag.category_for(tag)
+        if category_constraint
+          if category == category_constraint
+            counts[tag] += 1
+          end
+        else
+          counts[tag] += 1
+        end
+      end
+    end
+
+    counts
+  end
+
   def self.calculate_from_sample(tags, limit, category_constraint = nil)
     counts = Hash.new {|h, k| h[k] = 0}
 

app/presenters/post_set_presenters/post.rb

@@ -40,10 +40,14 @@ module PostSetPresenters
       if tag
         tag.related_tag_array.map(&:first)
       else
-        []
+        calculate_related_tags_from_post_set
       end
     end
 
+    def calculate_related_tags_from_post_set
+      RelatedTagCalculator.calculate_from_post_set_to_array(post_set).map(&:first)
+    end
+
     def tag_list_html(template)
       tag_set_presenter.tag_list_html(template)
     end