fix: Use fallback when titles are unavailable for outlines.

The Studio UI prevents you from creating a Section or Subsection
with no title (display_name). But OLX import allows you to bypass
these checks and create Sections ("chapter" tag) and Subsections
("sequential" tag) without display_name information specified in
the XML. When this happens, Studio and the LMS fall back to using
the url_name (the last part of the UsageKey) as a title, using the
display_name_with_default method.

This usually works, because url_names are derived from the import
file name, and if you're hand-editing a course in XML, your file
names are probably more intelligible than Mongo object IDs. In any
case, this commit updates get_outline_from_modulestore to match the
behavior of Studio and the LMS with respect to this situation.

This is part of the course outlines backfill rollout. TNL-8056

cms/djangoapps/contentstore/outlines.py

@@ -123,7 +123,7 @@ def _make_section_data(section):
         sequences_data.append(
             CourseLearningSequenceData(
                 usage_key=_remove_version_info(sequence.location),
-                title=sequence.display_name,
+                title=sequence.display_name_with_default,
                 inaccessible_after_due=sequence.hide_after_due,
                 exam=ExamData(
                     is_practice_exam=sequence.is_practice_exam,
@@ -139,7 +139,7 @@ def _make_section_data(section):
 
     section_data = CourseSectionData(
         usage_key=_remove_version_info(section.location),
-        title=section.display_name,
+        title=section.display_name_with_default,
         sequences=sequences_data,
         visibility=VisibilityData(
             hide_from_toc=section.hide_from_toc,
@@ -165,7 +165,7 @@ def get_outline_from_modulestore(course_key):
 
         course_outline_data = CourseOutlineData(
             course_key=course_key,
-            title=course.display_name,
+            title=course.display_name_with_default,
 
             # subtree_edited_on has a tzinfo of bson.tz_util.FixedOffset (which
             # maps to UTC), but for consistency, we're going to use the standard

cms/djangoapps/contentstore/tests/test_outlines.py

@@ -209,6 +209,26 @@ class OutlineFromModuleStoreTestCase(ModuleStoreTestCase):
         with self.assertRaises(CourseStructureError):
             get_outline_from_modulestore(self.course_key)
 
+    def test_missing_display_names(self):
+        """
+        When display_names are empty, it should fallback on url_names.
+        """
+        with self.store.bulk_operations(self.course_key):
+            section = ItemFactory.create(
+                parent_location=self.draft_course.location,
+                category='chapter',
+                display_name=None,
+            )
+            sequence = ItemFactory.create(
+                parent_location=section.location,
+                category='sequential',
+                display_name=None,
+            )
+
+        outline = get_outline_from_modulestore(self.course_key)
+        assert outline.sections[0].title == section.url_name
+        assert outline.sections[0].sequences[0].title == sequence.url_name
+
     def _outline_seq_data(self, modulestore_seq):
         """
         (CourseLearningSequenceData, UsageKey) for a Modulestore sequence.