function getDocumentIdWithContentHash(string _documentContentSHA256) public view
    returns (uint) 
    {
        bytes32 documentContentSHA256Keccak256 = keccak256(_documentContentSHA256);
        for (uint i = 0; i < documentsCount; i++) {
           Document memory doc = documents[i];
           if (keccak256(doc.documentContentSHA256)==documentContentSHA256Keccak256) {
               return i;
           }
        }
        return 0;
    }