favorites: fix posts with incorrect fav_count fields.

There were about 4000 posts with an incorrect fav_count.

script/fixes/087_fix_post_fav_counts.rb

@@ -0,0 +1,26 @@
+#!/usr/bin/env ruby
+
+require_relative "base"
+
+with_confirmation do
+  # Fix posts that have a non-zero fav_count but no favorites.
+  records = Post.where("fav_count != 0").where.not(id: Favorite.select(:post_id).distinct)
+  puts "Fixing #{records.size} records"
+  records.update_all(fav_count: 0)
+end
+
+with_confirmation do
+  # Fix posts that have a fav_count inconsistent with the favorites table.
+  records = Post.find_by_sql(<<~SQL.squish)
+    UPDATE posts
+    SET fav_count = true_count
+    FROM (
+      SELECT post_id, COUNT(*) AS true_count
+      FROM favorites
+      GROUP BY post_id
+    ) true_counts
+    WHERE posts.id = post_id AND posts.fav_count != true_count
+    RETURNING posts.*
+  SQL
+  puts "Fixing #{records.size} records"
+end