refactor: read course publish date from overview, not block structure

The `get_course_last_published` function is used by CourseGraph to
determine whether or not a course should be dumped to Neo4j.
If the course hasn't been published since it was last dumped to
Neo4j, then it can be skipped (unless the override_cache option
is enabled).

The function was previously built using the BlockStructure
data model. While this worked fine in Production instances that
enable `block_structure.storage_backing_for_cache`, this
implementation did NOT work in development environments,
which do not use the BlockStrcture model.

Instead, we switch to using CourseOverview.modified to
approximate when a course was last published. This is method
has fewer moving parts and is universally available across
instances.

cms/djangoapps/coursegraph/management/commands/tests/test_dump_to_neo4j.py

@@ -220,6 +220,7 @@ class TestDumpToNeo4jCommand(TestDumpToNeo4jCommandBase):
             number_rollbacks=0,
         )
 
+    @mock.patch('cms.djangoapps.coursegraph.tasks.NodeMatcher')
     @mock.patch('cms.djangoapps.coursegraph.tasks.Graph', autospec=True)
     @override_settings(
         COURSEGRAPH_CONNECTION=dict(
@@ -231,12 +232,15 @@ class TestDumpToNeo4jCommand(TestDumpToNeo4jCommandBase):
             password="default-password",
         )
     )
-    def test_dump_to_neo4j_connection_defaults(self, mock_graph_class):
+    def test_dump_to_neo4j_connection_defaults(self, mock_graph_class, mock_matcher_class):
         """
         Test that user can override individual settings.COURSEGRAPH_CONNECTION parameters
         by passing them to `dump_to_neo4j`, whilst falling back to the ones that they
         don't override.
         """
+        self.setup_mock_graph(
+            mock_matcher_class, mock_graph_class
+        )
         call_command(
             'dump_to_neo4j',
             courses=self.course_strings[:1],
@@ -244,7 +248,8 @@ class TestDumpToNeo4jCommand(TestDumpToNeo4jCommandBase):
             secure=False,
             password="overridden-password",
         )
-        mock_graph_class.assert_called_once_with(
+        assert mock_graph_class.call_args.args == ()
+        assert mock_graph_class.call_args.kwargs == dict(
 
             # From settings:
             protocol='bolt',

cms/djangoapps/coursegraph/tasks.py

@@ -134,29 +134,26 @@ def get_command_last_run(course_key, graph):
 
 def get_course_last_published(course_key):
     """
-    We use the CourseStructure table to get when this course was last
-    published.
+    Approximately when was a course last published?
+
+    We use the 'modified' column in the CourseOverview table as a quick and easy
+    (although perhaps inexact) way of determining when a course was last
+    published. This works because CourseOverview rows are re-written upon
+    course publish.
+
     Args:
         course_key: a CourseKey
 
-    Returns: The datetime the course was last published at, converted into
-        text, or None, if there's no record of the last time this course
-        was published.
+    Returns: The datetime the course was last published at, stringified.
+        Uses Python's default str(...) implementation for datetimes, which
+        is sortable and similar to ISO 8601:
+        https://docs.python.org/3/library/datetime.html#datetime.date.__str__
     """
     # Import is placed here to avoid model import at project startup.
-    from xmodule.modulestore.django import modulestore
-    from openedx.core.djangoapps.content.block_structure.models import BlockStructureModel
-    from openedx.core.djangoapps.content.block_structure.exceptions import BlockStructureNotFound
+    from openedx.core.djangoapps.content.course_overviews.models import CourseOverview
 
-    store = modulestore()
-    course_usage_key = store.make_course_usage_key(course_key)
-    try:
-        structure = BlockStructureModel.get(course_usage_key)
-        course_last_published_date = str(structure.modified)
-    except BlockStructureNotFound:
-        course_last_published_date = None
-
-    return course_last_published_date
+    approx_last_published = CourseOverview.get_from_id(course_key).modified
+    return str(approx_last_published)
 
 
 def strip_branch_and_version(location):