message: Fix change of _course ObjectID
diff: @@ -113,6 +113,7 @@ class FileUnitMigration {
  }));
  fileUnitObj._id = new ObjectID(fileUnitObj._id);
  + fileUnitObj._course = new ObjectID(fileUnitObj._course);
  const unitAfterReplace = await mongoose.connection.collection('units')
  .findOneAndReplace({'_id': fileUnit._id}, fileUnitObj);
  