docs: def add_dataset_to_collection(dataset_id, collection_id, **kwargs):
      """
          Add a single dataset to a dataset collection.
      """
      collection_i = _get_collection(collection_id)
      collection_item = _get_collection_item(collection_id, dataset_id)
      if collection_item is not None:
          raise HydraError("Dataset Collection %s already contains dataset %s", collection_id, dataset_id)
  
      new_item = DatasetCollectionItem()
      new_item.dataset_id=dataset_id
      new_item.collection_id=collection_id
  
      collection_i.items.append(new_item)
  
      db.DBSession.flush()
  
      return 'OK'