display diff of containing aggregated journal

... instead of displaying the diff for the requested journal only

- this should provide the best forward compatibility of links
- even when aggregation timeframes are changed intermittently

app/controllers/journals_controller.rb

@@ -88,13 +88,15 @@ class JournalsController < ApplicationController
   end
 
   def diff
+    journal = Journal::AggregatedJournal.for_journal(@journal)
+
     if valid_diff?
       field = params[:field].parameterize.underscore.to_sym
-      from = @journal.details[field][0]
-      to = @journal.details[field][1]
+      from = journal.details[field][0]
+      to = journal.details[field][1]
 
       @diff = Redmine::Helpers::Diff.new(to, from)
-      @journable = @journal.journable
+      @journable = journal.journable
       respond_to do |format|
         format.html {}
         format.js { render partial: 'diff', locals: { diff: @diff } }

app/models/journal/aggregated_journal.rb

@@ -41,6 +41,12 @@
 #    be dropped
 class Journal::AggregatedJournal
   class << self
+    # Returns the aggregated journal that contains the specified (vanilla) journal.
+    def for_journal(vanilla_journal)
+      Journal::AggregatedJournal.aggregated_journals(journable: vanilla_journal.journable)
+                  .detect { |journal| journal.version >= vanilla_journal.version }
+    end
+
     def with_notes_id(notes_id)
       raw_journal = query_aggregated_journals
                       .where("#{table_name}.id = ?", notes_id)

spec/models/journal/aggregated_journal_spec.rb

@@ -120,6 +120,14 @@ describe Journal::AggregatedJournal, type: :model do
         expect(subject.first.successor).to be_nil
       end
 
+      it 'returns the single journal for both original journals' do
+        expect(described_class.for_journal work_package.journals.first)
+          .to be_equivalent_to_journal subject.first
+
+        expect(described_class.for_journal work_package.journals.second)
+          .to be_equivalent_to_journal subject.first
+      end
+
       context 'with a comment' do
         let(:notes) { 'This is why I changed it.' }
 
@@ -153,6 +161,19 @@ describe Journal::AggregatedJournal, type: :model do
           it 'has the second as successor of the first journal' do
             expect(subject.first.successor).to be_equivalent_to_journal subject.second
           end
+
+          it 'returns the same aggregated journal for the first two originals' do
+            expect(described_class.for_journal work_package.journals.first)
+              .to be_equivalent_to_journal subject.first
+
+            expect(described_class.for_journal work_package.journals.second)
+              .to be_equivalent_to_journal subject.first
+          end
+
+          it 'returns a different aggregated journal for the last original' do
+            expect(described_class.for_journal work_package.journals.last)
+              .to be_equivalent_to_journal subject.second
+          end
         end
 
         context 'adding another change without comment' do