Check that object is null in solidity mapping - ethereum

Question

I have this solidity mapping
mapping (string => Ticket) public myMapping;
I want to check if myMapping[key] exists or not. How can I check?

Answer 1

The entire storage space is virtually initialized to 0 (there is no undefined).
So you have to compare the value to the 0 value for your type.
For example, mapping[key] == address(0x0) or mapping[key] = bytes4(0x0).

Answer 2

There is no direct method to check whether the mapping has particular key. But you can check if mapping property has value or not. The following example considered that the Ticket is the struct with some property.
pragma solidity >=0.4.21 <0.6.0;
contract Test {
struct Ticket {
uint seatNumber;
}
mapping (string => Ticket) myMapping;
function isExists(string memory key) public view returns (bool) {
if(myMapping[key].seatNumber != 0){
return true;
}
return false;
}
function add(string memory key, uint seatNumber) public returns (bool){
myMapping[key].seatNumber = seatNumber;
return true;
}
}

Answer 3

pragma solidity ^0.8.0;
contract BookLibNew{
address public owner;
constructor() public{
owner = msg.sender;
}
modifier onlyOwner(){
require(msg.sender == owner);
_;
}
struct bookDet{
uint bookId;
string bookTitle;
string bookAuthor;
}
mapping (uint8 => bookDet) public bookLib;
function addBookLib(uint8 _bookId, string memory _bookTitle, string memory _bookAuthor)
public onlyOwner {
require(bookLib(_bookId) == false, "Error: Book already exists");
bookLib[_bookId].bookTitle = _bookTitle;
bookLib[_bookId].bookAuthor = _bookAuthor;
}
function readBookDetails(uint8 _bookId) public view returns(string memory, string memory){
return(bookLib[_bookId].bookTitle, bookLib[_bookId].bookAuthor);
}
}