def load_preview_module(request, preview_id, descriptor):
    """
    Return a preview XModule instantiated from the supplied descriptor.

    request: The active django request
    preview_id (str): An identifier specifying which preview this module is used for
    descriptor: An XModuleDescriptor
    """
    system = preview_module_system(request, preview_id, descriptor)
    try:
        module = descriptor.xmodule(system)
    except:
        log.debug("Unable to load preview module", exc_info=True)
        module = ErrorDescriptor.from_descriptor(descriptor, error_msg=exc_info_to_str(sys.exc_info())).xmodule(system)

    # cdodge: Special case
    if module.location.category == "static_tab":
        module.get_html = wrap_xmodule(module.get_html, module, "xmodule_tab_display.html")
    else:
        module.get_html = wrap_xmodule(module.get_html, module, "xmodule_display.html")

    # we pass a partially bogus course_id as we don't have the RUN information passed yet
    # through the CMS. Also the contentstore is also not RUN-aware at this point in time.
    module.get_html = replace_static_urls(
        module.get_html,
        getattr(module, "data_dir", module.location.course),
        course_id=module.location.org + "/" + module.location.course + "/BOGUS_RUN_REPLACE_WHEN_AVAILABLE",
    )

    module.get_html = save_module(module.get_html, module)

    return module

 def test_error_module_xml_rendering(self):
     descriptor = ErrorDescriptor.from_xml(
         self.valid_xml, self.system, self.org, self.course, self.error_msg)
     self.assertIsInstance(descriptor, ErrorDescriptor)
     descriptor.xmodule_runtime = self.system
     context_repr = self.system.render(descriptor, 'student_view').content
     self.assertIn(self.error_msg, context_repr)
     self.assertIn(repr(self.valid_xml), context_repr)

    def xblock_from_json(self, class_, usage_id, json_data, course_entry_override=None):
        if course_entry_override is None:
            course_entry_override = self.course_entry
        # most likely a lazy loader or the id directly
        definition = json_data.get('definition', {})
        definition_id = self.modulestore.definition_locator(definition)

        # If no usage id is provided, generate an in-memory id
        if usage_id is None:
            usage_id = LocalId()

        block_locator = BlockUsageLocator(
            version_guid=course_entry_override['_id'],
            usage_id=usage_id,
            course_id=course_entry_override.get('course_id'),
            branch=course_entry_override.get('branch')
        )

        kvs = SplitMongoKVS(
            definition,
            json_data.get('fields', {}),
            json_data.get('_inherited_settings'),
        )
        field_data = DbModel(kvs)

        try:
            module = self.construct_xblock_from_class(
                class_,
                field_data,
                ScopeIds(None, json_data.get('category'), definition_id, block_locator)
            )
        except Exception:
            log.warning("Failed to load descriptor", exc_info=True)
            return ErrorDescriptor.from_json(
                json_data,
                self,
                BlockUsageLocator(
                    version_guid=course_entry_override['_id'],
                    usage_id=usage_id
                ),
                error_msg=exc_info_to_str(sys.exc_info())
            )

        edit_info = json_data.get('edit_info', {})
        module.edited_by = edit_info.get('edited_by')
        module.edited_on = edit_info.get('edited_on')
        module.previous_version = edit_info.get('previous_version')
        module.update_version = edit_info.get('update_version')
        module.definition_locator = self.modulestore.definition_locator(definition)
        # decache any pending field settings
        module.save()

        # If this is an in-memory block, store it in this system
        if isinstance(block_locator.usage_id, LocalId):
            self.local_modules[block_locator] = module

        return module

    def load_item(self, location):
        """
        Return an XModule instance for the specified location
        """
        location = Location(location)
        json_data = self.module_data.get(location)
        if json_data is None:
            module = self.modulestore.get_item(location)
            if module is not None:
                # update our own cache after going to the DB to get cache miss
                self.module_data.update(module.runtime.module_data)
            return module
        else:
            # load the module and apply the inherited metadata
            try:
                category = json_data['location']['category']
                class_ = XModuleDescriptor.load_class(
                    category,
                    self.default_class
                )
                definition = json_data.get('definition', {})
                metadata = json_data.get('metadata', {})
                for old_name, new_name in getattr(class_, 'metadata_translations', {}).items():
                    if old_name in metadata:
                        metadata[new_name] = metadata[old_name]
                        del metadata[old_name]

                kvs = MongoKeyValueStore(
                    definition.get('data', {}),
                    definition.get('children', []),
                    metadata,
                )

                field_data = DbModel(kvs)
                scope_ids = ScopeIds(None, category, location, location)
                module = self.construct_xblock_from_class(class_, field_data, scope_ids)
                if self.cached_metadata is not None:
                    # parent container pointers don't differentiate between draft and non-draft
                    # so when we do the lookup, we should do so with a non-draft location
                    non_draft_loc = location.replace(revision=None)

                    # Convert the serialized fields values in self.cached_metadata
                    # to python values
                    metadata_to_inherit = self.cached_metadata.get(non_draft_loc.url(), {})
                    inherit_metadata(module, metadata_to_inherit)
                # decache any computed pending field settings
                module.save()
                return module
            except:
                log.warning("Failed to load descriptor", exc_info=True)
                return ErrorDescriptor.from_json(
                    json_data,
                    self,
                    json_data['location'],
                    error_msg=exc_info_to_str(sys.exc_info())
                )

    def xblock_from_json(self, class_, usage_id, json_data, course_entry_override=None):
        if course_entry_override is None:
            course_entry_override = self.course_entry
        # most likely a lazy loader but not the id directly
        definition = json_data.get("definition", {})
        metadata = json_data.get("metadata", {})

        block_locator = BlockUsageLocator(
            version_guid=course_entry_override["_id"],
            usage_id=usage_id,
            course_id=course_entry_override.get("course_id"),
            branch=course_entry_override.get("branch"),
        )

        kvs = SplitMongoKVS(
            definition,
            json_data.get("children", []),
            metadata,
            json_data.get("_inherited_metadata"),
            block_locator,
            json_data.get("category"),
        )
        model_data = DbModel(
            kvs,
            class_,
            None,
            SplitMongoKVSid(
                # DbModel req's that these support .url()
                block_locator,
                self.modulestore.definition_locator(definition),
            ),
        )

        try:
            module = class_(self, model_data)
        except Exception:
            log.warning("Failed to load descriptor", exc_info=True)
            if usage_id is None:
                usage_id = "MISSING"
            return ErrorDescriptor.from_json(
                json_data,
                self,
                BlockUsageLocator(version_guid=course_entry_override["_id"], usage_id=usage_id),
                error_msg=exc_info_to_str(sys.exc_info()),
            )

        module.edited_by = json_data.get("edited_by")
        module.edited_on = json_data.get("edited_on")
        module.previous_version = json_data.get("previous_version")
        module.update_version = json_data.get("update_version")
        module.definition_locator = self.modulestore.definition_locator(definition)
        # decache any pending field settings
        module.save()
        return module

    def load_item(self, location):
        """
        Return an XModule instance for the specified location
        """
        location = Location(location)
        json_data = self.module_data.get(location)
        if json_data is None:
            module = self.modulestore.get_item(location)
            if module is not None:
                # update our own cache after going to the DB to get cache miss
                self.module_data.update(module.system.module_data)
            return module
        else:
            # load the module and apply the inherited metadata
            try:
                category = json_data['location']['category']
                class_ = XModuleDescriptor.load_class(
                    category,
                    self.default_class
                )
                definition = json_data.get('definition', {})
                metadata = json_data.get('metadata', {})
                for old_name, new_name in class_.metadata_translations.items():
                    if old_name in metadata:
                        metadata[new_name] = metadata[old_name]
                        del metadata[old_name]

                kvs = MongoKeyValueStore(
                    definition.get('data', {}),
                    definition.get('children', []),
                    metadata,
                    location,
                    category
                )

                model_data = DbModel(kvs, class_, None, MongoUsage(self.course_id, location))
                model_data['category'] = category
                model_data['location'] = location
                module = class_(self, model_data)
                if self.cached_metadata is not None:
                    # parent container pointers don't differentiate between draft and non-draft
                    # so when we do the lookup, we should do so with a non-draft location
                    non_draft_loc = location.replace(revision=None)
                    metadata_to_inherit = self.cached_metadata.get(non_draft_loc.url(), {})
                    inherit_metadata(module, metadata_to_inherit)
                return module
            except:
                log.warning("Failed to load descriptor", exc_info=True)
                return ErrorDescriptor.from_json(
                    json_data,
                    self,
                    json_data['location'],
                    error_msg=exc_info_to_str(sys.exc_info())
                )

 def test_error_module_xml_rendering(self):
     descriptor = ErrorDescriptor.from_xml(
         self.valid_xml,
         self.system,
         CourseLocationManager(self.course_id),
         self.error_msg
     )
     self.assertIsInstance(descriptor, ErrorDescriptor)
     descriptor.xmodule_runtime = self.system
     context_repr = self.system.render(descriptor, STUDENT_VIEW).content
     self.assertIn(self.error_msg, context_repr)
     self.assertIn(repr(self.valid_xml), context_repr)

    def xblock_from_json(self, class_, usage_id, json_data, course_entry_override=None):
        if course_entry_override is None:
            course_entry_override = self.course_entry
        else:
            # most recent retrieval is most likely the right one for next caller (see comment above fn)
            self.course_entry["branch"] = course_entry_override["branch"]
            self.course_entry["course_id"] = course_entry_override["course_id"]
        # most likely a lazy loader or the id directly
        definition = json_data.get("definition", {})
        definition_id = self.modulestore.definition_locator(definition)

        # If no usage id is provided, generate an in-memory id
        if usage_id is None:
            usage_id = LocalId()

        block_locator = BlockUsageLocator(
            version_guid=course_entry_override["structure"]["_id"],
            usage_id=usage_id,
            course_id=course_entry_override.get("course_id"),
            branch=course_entry_override.get("branch"),
        )

        kvs = SplitMongoKVS(definition, json_data.get("fields", {}), json_data.get("_inherited_settings"))
        field_data = DbModel(kvs)

        try:
            module = self.construct_xblock_from_class(
                class_, ScopeIds(None, json_data.get("category"), definition_id, block_locator), field_data
            )
        except Exception:
            log.warning("Failed to load descriptor", exc_info=True)
            return ErrorDescriptor.from_json(
                json_data,
                self,
                BlockUsageLocator(version_guid=course_entry_override["structure"]["_id"], usage_id=usage_id),
                error_msg=exc_info_to_str(sys.exc_info()),
            )

        edit_info = json_data.get("edit_info", {})
        module.edited_by = edit_info.get("edited_by")
        module.edited_on = edit_info.get("edited_on")
        module.previous_version = edit_info.get("previous_version")
        module.update_version = edit_info.get("update_version")
        module.definition_locator = self.modulestore.definition_locator(definition)
        # decache any pending field settings
        module.save()

        # If this is an in-memory block, store it in this system
        if isinstance(block_locator.usage_id, LocalId):
            self.local_modules[block_locator] = module

        return module

    def test_error_module_from_descriptor(self):
        descriptor = MagicMock([XModuleDescriptor],
                               runtime=self.system,
                               location=self.location,
                               _field_data=self.valid_xml)

        error_descriptor = ErrorDescriptor.from_descriptor(
            descriptor, self.error_msg)
        self.assertIsInstance(error_descriptor, ErrorDescriptor)
        error_descriptor.xmodule_runtime = self.system
        context_repr = self.system.render(error_descriptor, 'student_view').content
        self.assertIn(self.error_msg, context_repr)
        self.assertIn(repr(descriptor), context_repr)

    def test_error_module_from_descriptor(self):
        descriptor = MagicMock(
            spec=XModuleDescriptor,
            runtime=self.system,
            location=self.location,
        )

        error_descriptor = ErrorDescriptor.from_descriptor(
            descriptor, self.error_msg)
        self.assertIsInstance(error_descriptor, ErrorDescriptor)
        error_descriptor.xmodule_runtime = self.system
        context_repr = self.system.render(error_descriptor, STUDENT_VIEW).content
        self.assertIn(self.error_msg, context_repr)
        self.assertIn(repr(descriptor), context_repr)

    def xblock_from_json(self, class_, usage_id, json_data, course_entry_override=None):
        if course_entry_override is None:
            course_entry_override = self.course_entry
        # most likely a lazy loader or the id directly
        definition = json_data.get('definition', {})

        block_locator = BlockUsageLocator(
            version_guid=course_entry_override['_id'],
            usage_id=usage_id,
            course_id=course_entry_override.get('course_id'),
            branch=course_entry_override.get('branch')
        )

        kvs = SplitMongoKVS(
            definition,
            json_data.get('fields', {}),
            json_data.get('_inherited_settings'),
            block_locator,
            json_data.get('category'))
        model_data = DbModel(kvs, class_, None,
            SplitMongoKVSid(
                # DbModel req's that these support .url()
                block_locator,
                self.modulestore.definition_locator(definition)))

        try:
            module = class_(self, model_data)
        except Exception:
            log.warning("Failed to load descriptor", exc_info=True)
            if usage_id is None:
                usage_id = "MISSING"
            return ErrorDescriptor.from_json(
                json_data,
                self,
                BlockUsageLocator(version_guid=course_entry_override['_id'],
                    usage_id=usage_id),
                error_msg=exc_info_to_str(sys.exc_info())
            )

        edit_info = json_data.get('edit_info', {})
        module.edited_by = edit_info.get('edited_by')
        module.edited_on = edit_info.get('edited_on')
        module.previous_version = edit_info.get('previous_version')
        module.update_version = edit_info.get('update_version')
        module.definition_locator = self.modulestore.definition_locator(definition)
        # decache any pending field settings
        module.save()
        return module

    def test_course_error(self):
        """
        Ensure the view still returns results even if get_courses() returns an ErrorDescriptor. The ErrorDescriptor
        should be filtered out.
        """

        error_descriptor = ErrorDescriptor.from_xml(
            '<course></course>',
            get_test_system(),
            CourseLocationManager(CourseLocator(org='org', course='course', run='run')),
            None
        )

        descriptors = [error_descriptor, self.empty_course, self.course]

        with patch('xmodule.modulestore.mixed.MixedModuleStore.get_courses', Mock(return_value=descriptors)):
            self.test_get()

def load_preview_module(request, preview_id, descriptor):
    """
    Return a preview XModule instantiated from the supplied descriptor.

    request: The active django request
    preview_id (str): An identifier specifying which preview this module is used for
    descriptor: An XModuleDescriptor
    """
    system = preview_module_system(request, preview_id, descriptor)
    try:
        module = descriptor.xmodule(system)
    except:
        log.debug("Unable to load preview module", exc_info=True)
        module = ErrorDescriptor.from_descriptor(
            descriptor,
            error_msg=exc_info_to_str(sys.exc_info())
        ).xmodule(system)

    # cdodge: Special case
    if module.location.category == 'static_tab':
        module.get_html = wrap_xmodule(
            module.get_html,
            module,
            "xmodule_tab_display.html",
        )
    else:
        module.get_html = wrap_xmodule(
            module.get_html,
            module,
            "xmodule_display.html",
        )

    module.get_html = replace_static_urls(
        module.get_html,
        getattr(module, 'data_dir', module.location.course),
        course_namespace=Location([module.location.tag, module.location.org, module.location.course, None, None])
    )

    module.get_html = save_module(
        module.get_html,
        module
    )

    return module

def load_preview_module(request, preview_id, descriptor):
    """
    Return a preview XModule instantiated from the supplied descriptor.

    request: The active django request
    preview_id (str): An identifier specifying which preview this module is used for
    descriptor: An XModuleDescriptor
    """
    system = preview_module_system(request, preview_id, descriptor)
    try:
        module = descriptor.xmodule(system)
    except:
        log.debug("Unable to load preview module", exc_info=True)
        module = ErrorDescriptor.from_descriptor(
            descriptor,
            error_msg=exc_info_to_str(sys.exc_info())
        ).xmodule(system)

    return module

    def test_has_staff_access_to_preview_mode(self):
        """
        Tests users have right access to content in preview mode.
        """
        course_key = self.course.id
        usage_key = self.course.scope_ids.usage_id
        chapter = ItemFactory.create(category="chapter", parent_location=self.course.location)
        overview = CourseOverview.get_from_id(course_key)
        test_system = get_test_system()

        ccx = CcxFactory(course_id=course_key)
        ccx_locator = CCXLocator.from_course_locator(course_key, ccx.id)

        error_descriptor = ErrorDescriptor.from_xml(
            u"<problem>ABC \N{SNOWMAN}</problem>",
            test_system,
            CourseLocationManager(course_key),
            "error msg"
        )
        # Enroll student to the course
        CourseEnrollmentFactory(user=self.student, course_id=self.course.id)

        modules = [
            self.course,
            overview,
            chapter,
            ccx_locator,
            error_descriptor,
            course_key,
            usage_key,
        ]
        # Course key is not None
        self.assertTrue(
            bool(access.has_staff_access_to_preview_mode(self.global_staff, obj=self.course, course_key=course_key))
        )

        for user in [self.global_staff, self.course_staff, self.course_instructor]:
            for obj in modules:
                self.assertTrue(bool(access.has_staff_access_to_preview_mode(user, obj=obj)))
                self.assertFalse(bool(access.has_staff_access_to_preview_mode(self.student, obj=obj)))

    def xblock_from_json(
        self, class_, course_key, block_key, json_data, course_entry_override=None, **kwargs
    ):
        if course_entry_override is None:
            course_entry_override = self.course_entry
        else:
            # most recent retrieval is most likely the right one for next caller (see comment above fn)
            self.course_entry = CourseEnvelope(course_entry_override.course_key, self.course_entry.structure)

        definition_id = json_data.get('definition')

        # If no usage id is provided, generate an in-memory id
        if block_key is None:
            block_key = BlockKey(json_data['block_type'], LocalId())

        if definition_id is not None and not json_data.get('definition_loaded', False):
            definition_loader = DefinitionLazyLoader(
                self.modulestore,
                course_key,
                block_key.type,
                definition_id,
                lambda fields: self.modulestore.convert_references_to_keys(
                    course_key, self.load_block_type(block_key.type),
                    fields, self.course_entry.structure['blocks'],
                )
            )
        else:
            definition_loader = None

        # If no definition id is provide, generate an in-memory id
        if definition_id is None:
            definition_id = LocalId()

        # Construct the Block Usage Locator:
        block_locator = course_key.make_usage_key(
            block_type=block_key.type,
            block_id=block_key.id,
        )

        converted_fields = self.modulestore.convert_references_to_keys(
            block_locator.course_key, class_, json_data.get('fields', {}), self.course_entry.structure['blocks'],
        )
        if block_key in self._parent_map:
            parent_key = self._parent_map[block_key]
            parent = course_key.make_usage_key(parent_key.type, parent_key.id)
        else:
            parent = None
        kvs = SplitMongoKVS(
            definition_loader,
            converted_fields,
            parent=parent,
            field_decorator=kwargs.get('field_decorator')
        )

        if InheritanceMixin in self.modulestore.xblock_mixins:
            field_data = inheriting_field_data(kvs)
        else:
            field_data = KvsFieldData(kvs)

        try:
            module = self.construct_xblock_from_class(
                class_,
                ScopeIds(None, block_key.type, definition_id, block_locator),
                field_data,
            )
        except Exception:
            log.warning("Failed to load descriptor", exc_info=True)
            return ErrorDescriptor.from_json(
                json_data,
                self,
                course_entry_override.course_key.make_usage_key(
                    block_type='error',
                    block_id=block_key.id
                ),
                error_msg=exc_info_to_str(sys.exc_info())
            )

        edit_info = json_data.get('edit_info', {})
        module._edited_by = edit_info.get('edited_by')
        module._edited_on = edit_info.get('edited_on')
        module.previous_version = edit_info.get('previous_version')
        module.update_version = edit_info.get('update_version')
        module.source_version = edit_info.get('source_version', None)
        module.definition_locator = DefinitionLocator(block_key.type, definition_id)
        # decache any pending field settings
        module.save()

        # If this is an in-memory block, store it in this system
        if isinstance(block_locator.block_id, LocalId):
            self.local_modules[block_locator] = module

        return module

    def load_item(self, location):
        """
        Return an XModule instance for the specified location
        """
        assert isinstance(location, Location)
        json_data = self.module_data.get(location)
        if json_data is None:
            module = self.modulestore.get_item(location)
            if module is not None:
                # update our own cache after going to the DB to get cache miss
                self.module_data.update(module.runtime.module_data)
            return module
        else:
            # load the module and apply the inherited metadata
            try:
                category = json_data['location']['category']
                class_ = self.load_block_type(category)


                definition = json_data.get('definition', {})
                metadata = json_data.get('metadata', {})
                for old_name, new_name in getattr(class_, 'metadata_translations', {}).items():
                    if old_name in metadata:
                        metadata[new_name] = metadata[old_name]
                        del metadata[old_name]

                children = [
                    location.course_key.make_usage_key_from_deprecated_string(childloc)
                    for childloc in definition.get('children', [])
                ]
                data = definition.get('data', {})
                if isinstance(data, basestring):
                    data = {'data': data}
                mixed_class = self.mixologist.mix(class_)
                if data is not None:
                    data = self._convert_reference_fields_to_keys(mixed_class, location.course_key, data)
                metadata = self._convert_reference_fields_to_keys(mixed_class, location.course_key, metadata)
                kvs = MongoKeyValueStore(
                    data,
                    children,
                    metadata,
                )

                field_data = KvsFieldData(kvs)
                scope_ids = ScopeIds(None, category, location, location)
                module = self.construct_xblock_from_class(class_, scope_ids, field_data)
                if self.cached_metadata is not None:
                    # parent container pointers don't differentiate between draft and non-draft
                    # so when we do the lookup, we should do so with a non-draft location
                    non_draft_loc = location.replace(revision=None)

                    # Convert the serialized fields values in self.cached_metadata
                    # to python values
                    metadata_to_inherit = self.cached_metadata.get(non_draft_loc.to_deprecated_string(), {})
                    inherit_metadata(module, metadata_to_inherit)
                # decache any computed pending field settings
                module.save()
                return module
            except:
                log.warning("Failed to load descriptor from %s", json_data, exc_info=True)
                return ErrorDescriptor.from_json(
                    json_data,
                    self,
                    location,
                    error_msg=exc_info_to_str(sys.exc_info())
                )

    def xblock_from_json(self, class_, course_key, block_key, block_data, course_entry_override=None, **kwargs):
        """
        Load and return block info.
        """
        if course_entry_override is None:
            course_entry_override = self.course_entry
        else:
            # most recent retrieval is most likely the right one for next caller (see comment above fn)
            self.course_entry = CourseEnvelope(course_entry_override.course_key, self.course_entry.structure)

        definition_id = block_data.definition

        # If no usage id is provided, generate an in-memory id
        if block_key is None:
            block_key = BlockKey(block_data.block_type, LocalId())

        convert_fields = lambda field: self.modulestore.convert_references_to_keys(
            course_key, class_, field, self.course_entry.structure['blocks'],
        )

        if definition_id is not None and not block_data.definition_loaded:
            definition_loader = DefinitionLazyLoader(
                self.modulestore,
                course_key,
                block_key.type,
                definition_id,
                convert_fields,
            )
        else:
            definition_loader = None

        # If no definition id is provide, generate an in-memory id
        if definition_id is None:
            definition_id = LocalId()

        # Construct the Block Usage Locator:
        block_locator = course_key.make_usage_key(
            block_type=block_key.type,
            block_id=block_key.id,
        )

        converted_fields = convert_fields(block_data.fields)
        converted_defaults = convert_fields(block_data.defaults)
        if block_key in self._parent_map:
            parent_key = self._parent_map[block_key]
            parent = course_key.make_usage_key(parent_key.type, parent_key.id)
        else:
            parent = None

        aside_fields = None

        # for the situation if block_data has no asides attribute
        # (in case it was taken from memcache)
        try:
            if block_data.asides:
                aside_fields = {block_key.type: {}}
                for aside in block_data.asides:
                    aside_fields[block_key.type].update(aside['fields'])
        except AttributeError:
            pass

        try:
            kvs = SplitMongoKVS(
                definition_loader,
                converted_fields,
                converted_defaults,
                parent=parent,
                aside_fields=aside_fields,
                field_decorator=kwargs.get('field_decorator')
            )

            if InheritanceMixin in self.modulestore.xblock_mixins:
                field_data = inheriting_field_data(kvs)
            else:
                field_data = KvsFieldData(kvs)

            module = self.construct_xblock_from_class(
                class_,
                ScopeIds(None, block_key.type, definition_id, block_locator),
                field_data,
                for_parent=kwargs.get('for_parent')
            )
        except Exception:  # pylint: disable=broad-except
            log.warning("Failed to load descriptor", exc_info=True)
            return ErrorDescriptor.from_json(
                block_data,
                self,
                course_entry_override.course_key.make_usage_key(
                    block_type='error',
                    block_id=block_key.id
                ),
                error_msg=exc_info_to_str(sys.exc_info())
            )

        edit_info = block_data.edit_info
        module._edited_by = edit_info.edited_by  # pylint: disable=protected-access
        module._edited_on = edit_info.edited_on  # pylint: disable=protected-access
        module.previous_version = edit_info.previous_version
        module.update_version = edit_info.update_version
        module.source_version = edit_info.source_version
#.........这里部分代码省略.........

    def xblock_from_json(self, class_, block_id, json_data, course_entry_override=None):
        if course_entry_override is None:
            course_entry_override = self.course_entry
        else:
            # most recent retrieval is most likely the right one for next caller (see comment above fn)
            self.course_entry['branch'] = course_entry_override['branch']
            self.course_entry['org'] = course_entry_override['org']
            self.course_entry['offering'] = course_entry_override['offering']
        # most likely a lazy loader or the id directly
        definition = json_data.get('definition', {})
        definition_id = self.modulestore.definition_locator(definition)

        # If no usage id is provided, generate an in-memory id
        if block_id is None:
            block_id = LocalId()

        block_locator = BlockUsageLocator(
            CourseLocator(
                version_guid=course_entry_override['structure']['_id'],
                org=course_entry_override.get('org'),
                offering=course_entry_override.get('offering'),
                branch=course_entry_override.get('branch'),
            ),
            block_type=json_data.get('category'),
            block_id=block_id,
        )

        converted_fields = self.modulestore.convert_references_to_keys(
            block_locator.course_key, class_, json_data.get('fields', {}), self.course_entry['structure']['blocks'],
        )
        kvs = SplitMongoKVS(
            definition,
            converted_fields,
            json_data.get('_inherited_settings'),
        )
        field_data = KvsFieldData(kvs)

        try:
            module = self.construct_xblock_from_class(
                class_,
                ScopeIds(None, json_data.get('category'), definition_id, block_locator),
                field_data,
            )
        except Exception:
            log.warning("Failed to load descriptor", exc_info=True)
            return ErrorDescriptor.from_json(
                json_data,
                self,
                BlockUsageLocator(
                    CourseLocator(version_guid=course_entry_override['structure']['_id']),
                    block_type='error',
                    block_id=block_id
                ),
                error_msg=exc_info_to_str(sys.exc_info())
            )

        edit_info = json_data.get('edit_info', {})
        module.edited_by = edit_info.get('edited_by')
        module.edited_on = edit_info.get('edited_on')
        module.previous_version = edit_info.get('previous_version')
        module.update_version = edit_info.get('update_version')
        module.definition_locator = definition_id
        # decache any pending field settings
        module.save()

        # If this is an in-memory block, store it in this system
        if isinstance(block_locator.block_id, LocalId):
            self.local_modules[block_locator] = module

        return module

    def xblock_from_json(self, class_, course_key, block_key, block_data, course_entry_override=None, **kwargs):
        """
        Load and return block info.
        """
        if course_entry_override is None:
            course_entry_override = self.course_entry