'Params:
course_id: Course ID for which the asset metadata is stored.
doc_id: ObjectId of MongoDB document
asset_md: Dict with asset types as keys and lists of storable asset metadata as values.'

@contract(course_id='CourseKey', asset_md=dict)
def __init__(self, course_id, doc_id, asset_md):

    self.course_id = course_id
    self._doc_id = doc_id
    self.asset_md = asset_md
