Post#expunge!: run callbacks when reparenting children.

* Set parent IDs with `update` instead of `update_column` /
  `update_all` when reparenting children. This fixes it so that new post
  versions are saved and the has_children flag is set on the new parent.

* Slightly simplify logic of update_children_on_destroy: the single
  child case is subsumed by the multi-child case.

app/models/post.rb

@@ -1314,17 +1314,14 @@ class Post < ApplicationRecord
     end
 
     def update_children_on_destroy
-      if children.size == 0
-        # do nothing
-      elsif children.size == 1
-        children.first.update_column(:parent_id, nil)
-      else
-        cached_children = children
-        eldest = cached_children[0]
-        siblings = cached_children[1..-1]
-        eldest.update_column(:parent_id, nil)
-        Post.where(:id => siblings.map(&:id)).update_all(:parent_id => eldest.id)
-      end
+      return unless children.present?
+
+      eldest = children[0]
+      siblings = children[1..-1]
+
+      eldest.update(parent_id: nil)
+      Post.where(id: siblings).find_each { |p| p.update(parent_id: eldest.id) }
+      # Post.where(id: siblings).update(parent_id: eldest.id) # XXX rails 5
     end
 
     def update_parent_on_save

test/unit/post_test.rb

@@ -254,18 +254,39 @@ class PostTest < ActiveSupport::TestCase
       end
 
       context "two or more children" do
+        setup do
+          # ensure initial post versions won't be merged.
+          travel_to(1.day.ago) do
+            @p1 = FactoryGirl.create(:post)
+            @c1 = FactoryGirl.create(:post, :parent_id => @p1.id)
+            @c2 = FactoryGirl.create(:post, :parent_id => @p1.id)
+            @c3 = FactoryGirl.create(:post, :parent_id => @p1.id)
+          end
+        end
+
         should "reparent all children to the first child" do
-          p1 = FactoryGirl.create(:post)
-          c1 = FactoryGirl.create(:post, :parent_id => p1.id)
-          c2 = FactoryGirl.create(:post, :parent_id => p1.id)
-          c3 = FactoryGirl.create(:post, :parent_id => p1.id)
-          p1.expunge!
-          c1.reload
-          c2.reload
-          c3.reload
-          assert_nil(c1.parent_id)
-          assert_equal(c1.id, c2.parent_id)
-          assert_equal(c1.id, c3.parent_id)
+          @p1.expunge!
+          @c1.reload
+          @c2.reload
+          @c3.reload
+
+          assert_nil(@c1.parent_id)
+          assert_equal(@c1.id, @c2.parent_id)
+          assert_equal(@c1.id, @c3.parent_id)
+        end
+
+        should "save a post version record for each child" do
+          assert_difference(["@c1.versions.count", "@c2.versions.count", "@c3.versions.count"]) do
+            @p1.expunge!
+            @c1.reload
+            @c2.reload
+            @c3.reload
+          end
+        end
+
+        should "set the has_children flag on the new parent" do
+          @p1.expunge!
+          assert_equal(true, @c1.reload.has_children?)
         end
       end
     end