def get_op_w_rxns(db, operator_id):
    """Returns operator with all its associated reactions.

    :param db: DB to search
    :type db: A Mongo Database
    :param operator_id: Mongo _id or operator name (e.g. 1.1.-1.h)
    :type operator_id: str
    :return: Operator JSON document (with reactions)
    :rtype: list
    """
    operator = db.operators.find_one({'$or': [{'_id': operator_id}, {'Name':
        operator_id}]})
    if operator:
        operator['Reaction_ids'] = db.reactions.find({'Operators': operator_id}
            ).distinct('_id')
    else:
        return None
    return operator
