Solidity – How to Deal with Unique Off-Chain Metadata in Lazy Minting Contract

According to https://blog.openzeppelin.com/workshop-recap-building-an-nft-merkle-drop/

Lazy minting uses cryptographic primitives, the artist can sign “minting authorizations” that later allow a user to do the minting themselves. These signatures are free to produce, as they do not require an on-chain transaction. They guarantee that the artist or system administrator keeps total control over the NFTs, and ensure no token can be minted without prior approval.

You did not set the TokenURI for NFT, so when NFT gets the URI, BaseURI + TokenID is used.Look at the source code.

/**
 * @dev See {IERC721Metadata-tokenURI}.
 */
function tokenURI(uint256 tokenId) public view virtual override returns (string memory) {
    require(_exists(tokenId), "ERC721Metadata: URI query for nonexistent token");

    string memory _tokenURI = _tokenURIs[tokenId];
    string memory base = baseURI();

    // If there is no base URI, return the token URI.
    if (bytes(base).length == 0) {
        return _tokenURI;
    }
    // If both are set, concatenate the baseURI and tokenURI (via abi.encodePacked).
    if (bytes(_tokenURI).length > 0) {
        return string(abi.encodePacked(base, _tokenURI));
    }
    // If there is a baseURI but no tokenURI, concatenate the tokenID to the baseURI.
    return string(abi.encodePacked(base, tokenId.toString()));
}

It returns the URI: https://gateway.pinata.cloud/ipfs/0

This, of course, is not ideal, the ideal URI should be: https://gateway.pinata.cloud/ipfs/Qmf43nycxRXdwdq3GqT7SKU3MvRyBvbpjzufzZR8ASkMGb

I think you see the reason for the problem here.

The next thing you need to do is call _setTokenUri to store the CID from IPFS so that you can get the correct IPFS link when reading the NFT.

 /**
 * @dev Sets `_tokenURI` as the tokenURI of `tokenId`.
 *
 * Requirements:
 *
 * - `tokenId` must exist.
 */
function _setTokenURI(uint256 tokenId, string memory _tokenURI) internal virtual {
    require(_exists(tokenId), "ERC721Metadata: URI set of nonexistent token");
    _tokenURIs[tokenId] = _tokenURI;
}

This function is an internal function, and you should provide an external function to use it, but security must be taken care of, or anyone can modify it!