test: Update a BlockStructureFactory test mixin.

Previously, we were not caching BlockStructures to the database when we
were adding them to the store by default.  Now that we are doing that,
the BlockStructureFactory test failed because it was taking a shortcut
that would no longer work.  It was just creating a blockstructure that
had relations but not any block information.

Now that we're always persisting updates to the database, this broke
because to persist the structure to the database, we have to look up the
block information from the block_structure which now fails.

This change updates the test mixin to add more data so that the content
can be persisted to the database successfully as a part of this test.

openedx/core/djangoapps/content/block_structure/tests/helpers.py

@@ -274,10 +274,14 @@ class ChildrenMapTestMixin:
         # create empty block structure
         block_structure = block_structure_cls(root_block_usage_key=self.block_key_factory(0))
 
-        # _add_relation
+        # _add_relation and blocks
         for parent, children in enumerate(children_map):
+            if isinstance(block_structure, BlockStructureBlockData):
+                block_structure._get_or_create_block(self.block_key_factory(parent))  # pylint: disable=protected-access
             for child in children:
                 block_structure._add_relation(self.block_key_factory(parent), self.block_key_factory(child))  # pylint: disable=protected-access
+                if isinstance(block_structure, BlockStructureBlockData):
+                    block_structure._get_or_create_block(self.block_key_factory(child))  # pylint: disable=protected-access
         return block_structure
 
     def get_parents_map(self, children_map):

openedx/core/djangoapps/content/block_structure/tests/test_factory.py

@@ -5,6 +5,7 @@ Tests for block_structure_factory.py
 import pytest
 from django.test import TestCase
 
+from opaque_keys.edx.keys import CourseKey
 from xmodule.modulestore.exceptions import ItemNotFoundError
 
 from ..exceptions import BlockStructureNotFound
@@ -18,14 +19,22 @@ class TestBlockStructureFactory(TestCase, ChildrenMapTestMixin):
     Tests for BlockStructureFactory
     """
 
+    def block_key_factory(self, block_id):
+        """
+        Returns a usage_key object for the given block_id.
+        This overrides the method in the ChildrenMapTestMixin.
+        """
+        return CourseKey.from_string("course-v1:org+course+run").make_usage_key("html", str(block_id))
+
     def setUp(self):
         super().setUp()
         self.children_map = self.SIMPLE_CHILDREN_MAP
         self.modulestore = MockModulestoreFactory.create(self.children_map, self.block_key_factory)
 
     def test_from_modulestore(self):
+        usage_key = CourseKey.from_string("course-v1:org+course+run").make_usage_key("html", "0")
         block_structure = BlockStructureFactory.create_from_modulestore(
-            root_block_usage_key=0, modulestore=self.modulestore
+            root_block_usage_key=usage_key, modulestore=self.modulestore
         )
         self.assert_block_structure(block_structure, self.children_map)
 
@@ -48,15 +57,18 @@ class TestBlockStructureFactory(TestCase, ChildrenMapTestMixin):
 
     def test_from_cache_none(self):
         store = BlockStructureStore(MockCache())
+        # Non-existent usage key
+        usage_key = CourseKey.from_string("course-v1:org+course+run").make_usage_key("html", "0")
         with pytest.raises(BlockStructureNotFound):
             BlockStructureFactory.create_from_store(
-                root_block_usage_key=0,
+                root_block_usage_key=usage_key,
                 block_structure_store=store,
             )
 
     def test_new(self):
+        usage_key = CourseKey.from_string("course-v1:org+course+run").make_usage_key("html", "0")
         block_structure = BlockStructureFactory.create_from_modulestore(
-            root_block_usage_key=0, modulestore=self.modulestore
+            root_block_usage_key=usage_key, modulestore=self.modulestore
         )
         new_structure = BlockStructureFactory.create_new(
             block_structure.root_block_usage_key,