Rewrite migration for participation counter

This migrations runs only if the old 20150404193023_participation_counter
migration has not been run.

The new migration is a lot faster, because it runs after the
participation-cleanup migration has been run, so the participation table
is now a lot smaller for people upgrading from 0.5.x to 0.6.x.

It also doesn't remove manually created participations (without likes,
comments or poll participations on the same post).

Fixes #6786

db/migrate/20150404193023_participation_counter.rb

@@ -1,29 +0,0 @@
-class ParticipationCounter < ActiveRecord::Migration
-  def up
-    add_column :participations, :count, :int, null: false, default: 1
-
-    posts_count = Post.select("COUNT(posts.id)")
-                  .where("posts.id = participations.target_id")
-                  .where("posts.author_id = participations.author_id")
-                  .to_sql
-    likes_count = Like.select("COUNT(likes.id)")
-                  .where("likes.target_id = participations.target_id")
-                  .where("likes.author_id = participations.author_id")
-                  .to_sql
-    comments_count = Comment.select("COUNT(comments.id)")
-                     .where("comments.commentable_id = participations.target_id")
-                     .where("comments.author_id = participations.author_id")
-                     .to_sql
-    polls_count = PollParticipation.select("COUNT(*)")
-                  .where("poll_participations.author_id = participations.author_id")
-                  .joins(:poll)
-                  .where("polls.status_message_id = participations.target_id")
-                  .to_sql
-    Participation.update_all("count = (#{posts_count}) + (#{likes_count}) + (#{comments_count}) + (#{polls_count})")
-    Participation.where(count: 0).delete_all
-  end
-
-  def down
-    remove_column :participations, :count
-  end
-end

db/migrate/20160807212443_participation_counter.rb

@@ -0,0 +1,47 @@
+class ParticipationCounter < ActiveRecord::Migration
+  class Comment < ActiveRecord::Base
+  end
+
+  class Like < ActiveRecord::Base
+  end
+
+  class Participation < ActiveRecord::Base
+    belongs_to :author, class_name: "Person"
+  end
+
+  class Poll < ActiveRecord::Base
+  end
+
+  class PollParticipation < ActiveRecord::Base
+    belongs_to :poll
+  end
+
+  def up
+    return if ActiveRecord::SchemaMigration.where(version: "20150404193023").exists?
+
+    add_column :participations, :count, :integer, null: false, default: 1
+
+    likes_count = Like.select("COUNT(likes.id)")
+                      .where("likes.target_id = participations.target_id")
+                      .where("likes.author_id = participations.author_id")
+                      .to_sql
+    comments_count = Comment.select("COUNT(comments.id)")
+                            .where("comments.commentable_id = participations.target_id")
+                            .where("comments.author_id = participations.author_id")
+                            .to_sql
+    polls_count = PollParticipation.select("COUNT(*)")
+                                   .where("poll_participations.author_id = participations.author_id")
+                                   .joins(:poll)
+                                   .where("polls.status_message_id = participations.target_id")
+                                   .to_sql
+    Participation.joins(:author).where.not(people: {owner_id: nil})
+                 .update_all("count = (#{likes_count}) + (#{comments_count}) + (#{polls_count})")
+    Participation.where(count: 0).update_all(count: 1)
+  end
+
+  def down
+    remove_column :participations, :count
+
+    ActiveRecord::SchemaMigration.where(version: "20150404193023").delete_all
+  end
+end

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20160802212635) do
+ActiveRecord::Schema.define(version: 20160807212443) do
 
   create_table "account_deletions", force: :cascade do |t|
     t.string   "diaspora_handle", limit: 255