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/functional/explore/posts_controller_test.rb

@@ -22,7 +22,7 @@ module Explore
       context "#curated" do
         should "render" do
           @builder = create(:builder_user)
-          @post.add_favorite!(@builder)
+          Favorite.create!(post: @post, user: @builder)
           get curated_explore_posts_path
           assert_response :success
         end

test/functional/favorites_controller_test.rb

@@ -6,7 +6,7 @@ class FavoritesControllerTest < ActionDispatch::IntegrationTest
       @user = create(:user)
       @post = create(:post)
       @faved_post = create(:post)
-      @faved_post.add_favorite!(@user)
+      create(:favorite, post: @faved_post, user: @user)
     end
 
     context "index action" do
@@ -33,30 +33,48 @@ class FavoritesControllerTest < ActionDispatch::IntegrationTest
 
     context "create action" do
       should "create a favorite for the current user" do
-        assert_difference("Favorite.count", 1) do
+        assert_difference [-> { @post.favorites.count }, -> { @post.reload.fav_count }, -> { @user.reload.favorite_count }], 1 do
+          post_auth favorites_path(post_id: @post.id), @user, as: :javascript
+          assert_response :redirect
+        end
+      end
+
+      should "not allow creating duplicate favorites" do
+        create(:favorite, post: @post, user: @user)
+
+        assert_no_difference [-> { @post.favorites.count }, -> { @post.reload.fav_count }, -> { @user.reload.favorite_count }] do
           post_auth favorites_path(post_id: @post.id), @user, as: :javascript
           assert_response :redirect
         end
       end
 
       should "allow banned users to create favorites" do
-        assert_difference("Favorite.count", 1) do
-          post_auth favorites_path(post_id: @post.id), create(:banned_user), as: :javascript
+        @banned_user = create(:banned_user)
+
+        assert_difference [-> { @post.favorites.count }, -> { @post.reload.fav_count }, -> { @banned_user.reload.favorite_count }], 1 do
+          post_auth favorites_path(post_id: @post.id), @banned_user, as: :javascript
           assert_response :redirect
         end
       end
+
+      should "not allow anonymous users to create favorites" do
+        assert_no_difference [-> { @post.favorites.count }, -> { @post.reload.fav_count }] do
+          post favorites_path(post_id: @post.id), as: :javascript
+          assert_response 403
+        end
+      end
     end
 
     context "destroy action" do
-      should "remove the favorite from the current user" do
-        assert_difference("Favorite.count", -1) do
+      should "remove the favorite for the current user" do
+        assert_difference [-> { @faved_post.favorites.count }, -> { @faved_post.reload.fav_count }, -> { @user.reload.favorite_count }], -1 do
           delete_auth favorite_path(@faved_post.id), @user, as: :javascript
           assert_response :redirect
         end
       end
 
       should "allow banned users to destroy favorites" do
-        assert_difference("Favorite.count", -1) do
+        assert_difference [-> { @faved_post.favorites.count }, -> { @faved_post.reload.fav_count }, -> { @user.reload.favorite_count }], -1 do
           delete_auth favorite_path(@faved_post.id), @user, as: :javascript
           assert_response :redirect
         end

test/functional/moderator/post/posts_controller_test.rb

@@ -34,7 +34,7 @@ module Moderator
             end
             users = FactoryBot.create_list(:user, 2)
             users.each do |u|
-              @child.add_favorite!(u)
+              Favorite.create!(post: @child, user: u)
               @child.reload
             end