posts: stop updating fav_string attribute.

Stop updating the fav_string attribute on posts. The column still exists
on the table, but is no longer used or updated.

Like the pool_string in 7d503f08, the fav_string was used in the past to
facilitate `fav:X` searches. Posts had a hidden fav_string column that
contained a list of every user who favorited the post. These were
treated like fake hidden tags on the post so that a search for `fav:X`
was treated like a tag search.

The fav_string attribute has been unused for search purposes for a while
now. It was only kept because of technicalities that required
departitioning the favorites table first (340e1008e) before it could be
removed. Basically, removing favorites with `@favorite.destroy` was
slow because Rails always deletes object by ID, but we didn't have an
index on favorites.id, and we couldn't easily add one until the
favorites table was departitioned.

Fixes #4652. See https://github.com/danbooru/danbooru/issues/4652#issuecomment-754993802
for more discussion of issues caused by the fav_string (in short: write
amplification, post table bloat, and favorite inconsistency problems).

test/unit/favorite_test.rb

@@ -2,50 +2,95 @@ require 'test_helper'
 
 class FavoriteTest < ActiveSupport::TestCase
   setup do
-    @user1 = FactoryBot.create(:user)
-    @user2 = FactoryBot.create(:user)
-    @p1 = FactoryBot.create(:post)
-    @p2 = FactoryBot.create(:post)
-
-    CurrentUser.user = @user1
-    CurrentUser.ip_addr = "127.0.0.1"
+    @user1 = create(:user)
+    @user2 = create(:user)
+    @p1 = create(:post)
+    @p2 = create(:post)
   end
 
-  teardown do
-    CurrentUser.user = nil
-    CurrentUser.ip_addr = nil
-  end
+  context "Favorites: " do
+    context "removing a favorite" do
+      should "update the post and user favorite counts" do
+        fav = Favorite.create!(post: @p1, user: @user1)
 
-  context "A favorite" do
-    should "delete from all tables" do
-      @p1.add_favorite!(@user1)
-      assert_equal(1, @user1.favorite_count)
+        assert_equal(1, @user1.reload.favorite_count)
+        assert_equal(1, @p1.reload.fav_count)
 
-      Favorite.where(:user_id => @user1.id, :post_id => @p1.id).delete_all
-      assert_equal(0, Favorite.count)
+        Favorite.destroy_by(post: @p1, user: @user1)
+
+        assert_equal(0, @user1.reload.favorite_count)
+        assert_equal(0, @p1.reload.fav_count)
+      end
+
+      should "remove the upvote if the user could vote" do
+        @user = create(:gold_user)
+        @vote = create(:post_vote, post: @p1, user: @user, score: 1)
+        fav = Favorite.create!(post: @p1, user: @user)
+
+        assert_equal(1, @user.reload.favorite_count)
+        assert_equal(1, @p1.reload.fav_count)
+        assert_equal(1, @p1.reload.score)
+        assert(PostVote.positive.exists?(post: @p1, user: @user))
+
+        Favorite.destroy_by(post: @p1, user: @user)
+
+        assert_equal(0, @user.reload.favorite_count)
+        assert_equal(0, @p1.reload.fav_count)
+        assert_equal(0, @p1.reload.score)
+        refute(PostVote.positive.exists?(post: @p1, user: @user))
+      end
     end
 
-    should "know which table it belongs to" do
-      @p1.add_favorite!(@user1)
-      @p2.add_favorite!(@user1)
-      @p1.add_favorite!(@user2)
+    context "adding a favorite" do
+      should "update the post and user favorite counts" do
+        Favorite.create!(post: @p1, user: @user1)
 
-      favorites = @user1.favorites.order("id desc")
-      assert_equal(2, favorites.count)
-      assert_equal(@p2.id, favorites[0].post_id)
-      assert_equal(@p1.id, favorites[1].post_id)
+        assert_equal(1, @user1.reload.favorite_count)
+        assert_equal(1, @p1.reload.fav_count)
+      end
 
-      favorites = @user2.favorites.order("id desc")
-      assert_equal(1, favorites.count)
-      assert_equal(@p1.id, favorites[0].post_id)
-    end
+      should "not upvote the post if the user can't vote" do
+        Favorite.create!(post: @p1, user: @user1)
 
-    should "not allow duplicates" do
-      @p1.add_favorite!(@user1)
-      error = assert_raises(Favorite::Error) { @p1.add_favorite!(@user1) }
+        assert_equal(1, @user1.reload.favorite_count)
+        assert_equal(1, @p1.reload.fav_count)
+        assert_equal(0, @p1.reload.score)
+        refute(PostVote.positive.exists?(post: @p1, user: @user1))
+      end
 
-      assert_equal("You have already favorited this post", error.message)
-      assert_equal(1, @user1.favorite_count)
+      should "upvote the post if the user can vote" do
+        @user = create(:gold_user)
+        Favorite.create!(post: @p1, user: @user)
+
+        assert_equal(1, @user.reload.favorite_count)
+        assert_equal(1, @p1.reload.fav_count)
+        assert_equal(1, @p1.reload.score)
+        assert(PostVote.positive.exists?(post: @p1, user: @user))
+      end
+
+      should "convert a downvote into an upvote if the post was downvoted" do
+        @user = create(:gold_user)
+        @vote = create(:post_vote, post: @p1, user: @user, score: -1)
+
+        assert_equal(-1, @p1.reload.score)
+        Favorite.create!(post: @p1, user: @user)
+
+        assert_equal(1, @user.reload.favorite_count)
+        assert_equal(1, @p1.reload.fav_count)
+        assert_equal(1, @p1.reload.score)
+        assert(PostVote.positive.exists?(post: @p1, user: @user))
+        refute(PostVote.negative.exists?(post: @p1, user: @user))
+      end
+
+      should "not allow duplicate favorites" do
+        @f1 = Favorite.create(post: @p1, user: @user1)
+        @f2 = Favorite.create(post: @p1, user: @user1)
+
+        assert_equal(["You have already favorited this post"], @f2.errors.full_messages)
+        assert_equal(1, @user1.reload.favorite_count)
+        assert_equal(1, @p1.reload.fav_count)
+        assert_equal(0, @p1.reload.score)
+      end
     end
   end
 end