Add make_course_key to modulestores, so that external clients don't need to assume a CourseKey subtype.

Conflicts:
	common/lib/xmodule/xmodule/modulestore/xml_importer.py

common/lib/xmodule/xmodule/modulestore/mixed.py

@@ -170,6 +170,15 @@ class MixedModuleStore(ModuleStoreDraftAndPublished, ModuleStoreWriteBase):
 
         # return the default store
         return self.default_modulestore
+        # return the first store, as the default
+        return self.default_modulestore
+
+    @property
+    def default_modulestore(self):
+        """
+        Return the default modulestore
+        """
+        return self.modulestores[0]
 
     def _get_modulestore_by_type(self, modulestore_type):
         """

common/lib/xmodule/xmodule/modulestore/tests/test_cross_modulestore_import_export.py

@@ -235,7 +235,10 @@ MODULESTORE_SETUPS = (
     MixedModulestoreBuilder([('split', VersioningModulestoreBuilder())]),
 )
 CONTENTSTORE_SETUPS = (MongoContentstoreBuilder(),)
-COURSE_DATA_NAMES = ('toy', 'manual-testing-complete')
+COURSE_DATA_NAMES = (
+    'toy',
+    'manual-testing-complete',
+)
 
 
 @ddt.ddt
@@ -259,8 +262,6 @@ class CrossStoreXMLRoundtrip(CourseComparisonTest):
     ))
     @ddt.unpack
     def test_round_trip(self, source_builder, dest_builder, source_content_builder, dest_content_builder, course_data_name):
-        source_course_key = SlashSeparatedCourseKey('source', 'course', 'key')
-        dest_course_key = SlashSeparatedCourseKey('dest', 'course', 'key')
 
         # Construct the contentstore for storing the first import
         with source_content_builder.build() as source_content:
@@ -270,6 +271,9 @@ class CrossStoreXMLRoundtrip(CourseComparisonTest):
                 with dest_content_builder.build() as dest_content:
                     # Construct the modulestore for storing the second import (using the second contentstore)
                     with dest_builder.build(dest_content) as dest_store:
+                        source_course_key = source_store.make_course_key('source', 'course', 'key')
+                        dest_course_key = dest_store.make_course_key('dest', 'course', 'key')
+
                         import_from_xml(
                             source_store,
                             'test_user',

common/lib/xmodule/xmodule/modulestore/xml_importer.py

@@ -168,7 +168,7 @@ def import_from_xml(
         if target_course_id is not None:
             dest_course_id = target_course_id
         else:
-            dest_course_id = course_key
+            dest_course_id = store.make_course_key(course_key.org, course_key.course, course_key.run)
 
         # Creates a new course if it doesn't already exist
         if create_new_course_if_not_present and not store.has_course(dest_course_id, ignore_case=True):