'hydrates the activities using the activity_storage'

def hydrate_activities(self, activities):

    activity_ids = []
    for activity in activities:
        activity_ids += activity._activity_ids
    activity_list = self.activity_storage.get_many(activity_ids)
    activity_data = {a.serialization_id: a for a in activity_list}
    return [activity.get_hydrated(activity_data) for activity in activities]
