Avoid overwrite of mapping

Question

I have this smart contract:

contract Truerec {
    struct Certificate {
        address issuerAddress;
        string  identityType;
        string  identityHash;
        string  dataHash;
        bool   revoked;
    }

    mapping(bytes32 => Certificate) certificates;

    function issue(string _key, 
        string _identityType, 
        string _identityHash, 
        string _dataHash) {
        certificates[sha3(_key)] = Certificate(
            msg.sender,
            _identityType,
            _identityHash,
            _dataHash,
            false
        );
    }
    ...
}

How can I check whether a mapping already exists and by using that make sure that if the key already exists, that the existing mapping will not be overwritten?

I need something like:

function issue(string _key, 
    string _identityType, 
    string _identityHash, 
    string _dataHash) {

    require(!key_exists(sha3(_key, certificates)]

    certificates[sha3(_key)] = Certificate(
        msg.sender,
        _identityType,
        _identityHash,
        _dataHash,
        false
    );
}

Answer 1

From: http://solidity.readthedocs.io/en/develop/types.html#mappings

Mappings can be seen as hashtables which are virtually initialized such that every possible key exists and is mapped to a value whose byte-representation is all zeros: a type’s default value.

A trick I have seen being used some times is to have a bool as the first field, as follows:

struct Certificate {
    bool exists;
    address issuerAddress;
    string  identityType;
    string  identityHash;
    string  dataHash;
    bool   revoked;
}

When a certificate is created, set the exists field to always be true. The default value of a bool is false. So the check to see whether a key is already mapped would be:

certificates[sha3(_key)].exists