Fix #3822: Move favorites (while deleting) fails if user has also favorited destination

app/models/post.rb

@@ -966,6 +966,13 @@ class Post < ApplicationRecord
       clean_fav_string! if clean_fav_string?
     end
 
+    def add_favorite(user)
+      add_favorite!(user)
+      true
+    rescue Favorite::Error
+      false
+    end
+
     def add_favorite!(user)
       Favorite.add(post: self, user: user)
       vote!("up", user) if user.is_voter?
@@ -1295,7 +1302,7 @@ class Post < ApplicationRecord
       transaction do
         favorites.each do |fav|
           remove_favorite!(fav.user)
-          parent.add_favorite!(fav.user)
+          parent.add_favorite(fav.user)
         end
       end
 

test/unit/post_test.rb

@@ -68,7 +68,6 @@ class PostTest < ActiveSupport::TestCase
       end
 
       should "decrement the user's favorite count" do
-        @post.add_favorite!(@post.uploader)
         assert_difference(["@post.uploader.reload.favorite_count"], -1) do
           @post.expunge!
         end
@@ -1720,13 +1719,13 @@ class PostTest < ActiveSupport::TestCase
         assert_equal(0, @post.score)
       end
 
-      should "update the fav strings ont he post" do
+      should "update the fav strings on the post" do
         @post.add_favorite!(@user)
         @post.reload
         assert_equal("fav:#{@user.id}", @post.fav_string)
         assert(Favorite.exists?(:user_id => @user.id, :post_id => @post.id))
 
-        @post.add_favorite!(@user)
+        assert_raises(Favorite::Error) { @post.add_favorite!(@user) }
         @post.reload
         assert_equal("fav:#{@user.id}", @post.fav_string)
         assert(Favorite.exists?(:user_id => @user.id, :post_id => @post.id))