Fix #3981: Inconsistency between posts and pools.

app/models/pool.rb

@@ -241,8 +241,9 @@ class Pool < ApplicationRecord
   end
 
   def synchronize
-    added = post_ids - post_ids_was
-    removed = post_ids_was - post_ids
+    post_ids_before = post_ids_before_last_save || post_ids_was
+    added = post_ids - post_ids_before
+    removed = post_ids_before - post_ids
 
     added.each do |post_id|
       post = Post.find(post_id)

test/unit/pool_test.rb

@@ -61,6 +61,13 @@ class PoolTest < ActiveSupport::TestCase
     should "initialize the post count" do
       assert_equal(@posts.size, @pool.post_count)
     end
+
+    should "synchronize the posts with the pool" do
+      assert_equal(@posts.map(&:id), @pool.post_ids)
+
+      @posts.each(&:reload)
+      assert_equal(["pool:#{@pool.id} pool:series"] * @posts.size, @posts.map(&:pool_string))
+    end
   end
 
   context "Reverting a pool" do