search: add comment_count, note_count metatags (#4004).

Add these metatags:

* comment_count
* deleted_comment_count
* active_comment_count
* note_count
* deleted_note_count
* active_note_count
* order:comment_count
* order:deleted_comment_count
* order:active_comment_count
* order:note_count
* order:deleted_note_count
* order:active_note_count

app/models/post.rb

@@ -1609,6 +1609,33 @@ class Post < ApplicationRecord
       joins("CROSS JOIN unnest(string_to_array(tag_string, ' ')) AS tag")
     end
 
+    def with_comment_stats
+      relation = left_outer_joins(:comments).group(:id).select("posts.*")
+      relation = relation.select("COUNT(comments.id) AS comment_count")
+      relation = relation.select("COUNT(comments.id) FILTER (WHERE comments.is_deleted = TRUE)  AS deleted_comment_count")
+      relation = relation.select("COUNT(comments.id) FILTER (WHERE comments.is_deleted = FALSE) AS active_comment_count")
+      relation
+    end
+
+    def with_note_stats
+      relation = left_outer_joins(:notes).group(:id).select("posts.*")
+      relation = relation.select("COUNT(notes.id) AS note_count")
+      relation = relation.select("COUNT(notes.id) FILTER (WHERE notes.is_active = TRUE)  AS active_note_count")
+      relation = relation.select("COUNT(notes.id) FILTER (WHERE notes.is_active = FALSE) AS deleted_note_count")
+      relation
+    end
+
+    def with_stats(tables)
+      return all if tables.empty?
+
+      relation = all
+      tables.each do |table|
+        relation = relation.send("with_#{table}_stats")
+      end
+
+      from(relation.arel.as("posts"))
+    end
+
     def pending
       where(is_pending: true)
     end

app/models/tag.rb

@@ -1,5 +1,9 @@
 class Tag < ApplicationRecord
   COSINE_SIMILARITY_RELATED_TAG_THRESHOLD = 300
+  COUNT_METATAGS = %w[
+    comment_count deleted_comment_count active_comment_count
+    note_count deleted_note_count active_note_count
+  ]
   METATAGS = %w[
     -user user -approver approver commenter comm noter noteupdater artcomm
     -pool pool ordpool -favgroup favgroup -fav fav ordfav md5 -rating rating
@@ -7,7 +11,7 @@ class Tag < ApplicationRecord
     -source id -id date age order limit -status status tagcount parent -parent
     child pixiv_id pixiv search upvote downvote filetype -filetype flagger
     -flagger appealer -appealer disapproval -disapproval
-  ] + TagCategory.short_name_list.map {|x| "#{x}tags"}
+  ] + TagCategory.short_name_list.map {|x| "#{x}tags"} + COUNT_METATAGS
 
   SUBQUERY_METATAGS = %w[commenter comm noter noteupdater artcomm flagger -flagger appealer -appealer]
 
@@ -790,6 +794,9 @@ class Tag < ApplicationRecord
               q[:downvote] = User.name_to_id(g2)
             end
 
+          when *COUNT_METATAGS
+            q[g1.to_sym] = parse_helper(g2)
+
           end
 
         else