MagnetoDB: If item exists

// insert if item doesn't exist
{
    "item": {
        "Author": {
            "S": "John Doe22"
        },
        "MessageId": {
            "S": "1"
        },
        "Body": {
            "S": "Some vacation issues"
        },
        "DateTime": {
          "N": "20140313164951"
        },
        "DateTime3": {
          "N": "20140313164951"
        }
    },
  "expected":{
        "Author":{
          "exists":false
        }

  },
  "return_values":"ALL_OLD"
}


// table description
{
    "table_name": "email_data",
    "attribute_definitions": [
        {
            "attribute_name": "Author",
            "attribute_type": "S"
        },
        {
            "attribute_name": "Subject",
            "attribute_type": "S"
        },
        {
            "attribute_name": "Body",
            "attribute_type": "S"
        },
        {
            "attribute_name": "DateTime",
            "attribute_type": "N"
        },
        {
            "attribute_name": "MessageId",
            "attribute_type": "S"
        }
    ],
    "key_schema": [
        {
            "attribute_name": "Author",
            "key_type": "HASH"
        },
        {
            "attribute_name": "MessageId",
            "key_type": "RANGE"
        }
    ],
    "local_secondary_indexes": [
        {
            "index_name": "Subject_index",
            "key_schema": [
                {
                    "attribute_name": "Author",
                    "key_type": "HASH"
                },
                {
                    "attribute_name": "Subject",
                    "key_type": "RANGE"
                }
            ],
            "projection": {
                "projection_type": "ALL"
            }
        },
        {
            "index_name": "DateTime_index",
            "key_schema": [
                {
                    "attribute_name": "Author",
                    "key_type": "HASH"
                },
                {
                    "attribute_name": "DateTime",
                    "key_type": "RANGE"
                }
            ],
            "projection": {
                "projection_type": "ALL"
            }
        }
    ]
}