[59539] Correctly reschedule parent when it no longer has children

But only if it has a predecessor. If it does not have any predecessor
nor child, then it can't stay in automatic mode.

app/services/work_packages/update_service.rb

@@ -129,17 +129,34 @@ def reset_custom_values(work_package)
   end
 
   def reschedule_related(work_package)
-    rescheduled = if work_package.saved_change_to_parent_id? && work_package.parent_id_before_last_save
-                    reschedule_former_siblings(work_package).dependent_results
-                  else
-                    []
-                  end
+    moved_work_packages = [work_package]
+
+    # if parent changed, we find a child or a predecessor of the former parent to
+    # give it to the SetScheduleService so that the former parent is rescheduled.
+    if parent_just_changed?(work_package)
+      former_parent_id = work_package.parent_id_before_last_save
+      some_child_or_predecessor = find_some_child_or_predecessor(former_parent_id)
+      if some_child_or_predecessor
+        moved_work_packages << some_child_or_predecessor
+      else # rubocop:disable Style/EmptyElse
+        # aha! switch former parent to manual mode?
+      end
+    end
 
-    rescheduled + reschedule(work_package, [work_package]).dependent_results
+    reschedule(work_package, moved_work_packages).dependent_results
   end
 
-  def reschedule_former_siblings(work_package)
-    reschedule(work_package, WorkPackage.where(parent_id: work_package.parent_id_before_last_save))
+  def parent_just_changed?(work_package)
+    work_package.saved_change_to_parent_id? && work_package.parent_id_before_last_save
+  end
+
+  def find_some_child_or_predecessor(former_parent_id)
+    former_parent = WorkPackage.find(former_parent_id)
+    if a_child = former_parent.children.first
+      a_child
+    elsif a_relation = Relation.follows.of_successor(former_parent).first
+      a_relation.predecessor
+    end
   end
 
   def reschedule(work_package, work_packages)

spec/services/work_packages/update_service_integration_spec.rb

@@ -1196,12 +1196,6 @@
 
     let(:attributes) { { parent: nil } }
 
-    before do
-      work_package.reload
-      parent_work_package.reload
-      sibling_work_package.reload
-    end
-
     it "removes the parent and reschedules it" do
       expect(subject)
         .to be_success
@@ -1220,6 +1214,37 @@
     end
   end
 
+  context "when removing the last child of an automatically scheduled parent" do
+    let(:attributes) { { parent: nil } }
+
+    describe "when the parent has predecessors and successors" do
+      let_work_packages(<<~TABLE)
+        hierarchy      | MTWTFSS | scheduling mode | predecessors
+        predecessor    | X       | manual          |
+        parent         |    XXX  | automatic       | predecessor
+          work_package |    XXX  | manual          |
+        successor      |       X | automatic       | parent
+      TABLE
+
+      it "keeps former parent duration and moves it to its soonest start date, and successors are rescheduled" do
+        expect(subject)
+          .to be_success
+        expect(subject.all_results.pluck(:subject))
+          .to contain_exactly("work_package", "parent", "successor")
+        expect(work_package.reload.parent).to be_nil
+
+        expect_work_packages(subject.all_results + [predecessor], <<~TABLE)
+          subject        | MTWTFSS | scheduling mode |
+          predecessor    | X       | manual          |
+          parent         |  XXX    | automatic       |
+          successor      |     X   | automatic       |
+
+          work_package   |    XXX  | manual          |
+        TABLE
+      end
+    end
+  end
+
   describe "replacing the attachments" do
     let!(:old_attachment) do
       create(:attachment, container: work_package)