'Get the definition from the persistence mechanism whose id is the given key'

def get_definition(self, key, course_context=None):

    with TIMER.timer('get_definition', course_context) as tagger:
        definition = self.definitions.find_one({'_id': key})
        tagger.measure('fields', len(definition['fields']))
        tagger.tag(block_type=definition['block_type'])
        return definition
