Which information hold an NFT?

Question

I am developing an NFT market using solidity, specifically I am creating my own smart contract on top of OpenZeppelin's ERC-721 smart contract. My NFT at the moment have 5 attributes (id, image, description, collection and image) for the image, I save the hash that ipfs develve when uploading it.

My question is where to save all these attributes, since I have the Image struct that has the aforementioned attributes, I add it to an array and I mint the NFT using the id of the Image object in the array and the address of the creator. I mean, I'm saving all the information outside of the ERC-721 contract, so I don't quite understand what an NFT is, since the attributes are not from the NFT but the NFT is an attribute of my struct.

Am I implementing it correctly and the ERC-721 standard is only the necessary functions of an NFT or am I saving the information where it does not touch?

My code is currently the following:

pragma solidity ^0.5.0;

import "./ERC721Full.sol";

contract NftShop is ERC721Full {
  string public name;
  Image[] public nft;
  uint public imageId = 0;
  mapping(uint => bool) public _nftExists;
  mapping(uint => Image) public images;

  struct Image {
    uint id;                  //id of the nft
    string hash;              //hash of the ipfs            
    string description;       //nft description
    string collection;        //what collection the nft bellongs
    address payable author;   //creator of the nft
  }

  //Event used when new Token is created
  event TokenCreated(
    uint id,
    string hash,
    string description,
    string collection,
    address payable author
  );

  constructor() public payable ERC721Full("NftShop", "NFTSHOP") {
    name = "NftShop";
  }

  //uploadImage to the blockchain and mint the nft.
  function uploadImage(string memory _imgHash, string memory _description, string memory _collection) public {
    // Make sure the image hash exists
    require(bytes(_imgHash).length > 0);
    // Make sure image description exists
    require(bytes(_description).length > 0);
    // Make sure collectionage exists
    require(bytes(_collection).length > 0);
    // Make sure uploader address exists
    require(msg.sender!=address(0));

    // Increment image id
    imageId ++;

    // Add Image to the contract
    images[imageId] = Image(imageId, _imgHash, _description, _collection, msg.sender);

    //Mint the token
    require(!_nftExists[imageId]);
    uint _id = nft.push(images[imageId]);
    _mint(msg.sender, _id);
    _nftExists[imageId] = true;

    // Trigger an event
    emit TokenCreated(imageId, _imgHash, _description, _collection, msg.sender);
  }
} 

Any suggestions on how to improve the code if there is something weird is welcome.

I hope it is not an absurd question, I am starting in the world of Ethereum.

Thanks a lot.

Answer 1

Yes the information such as image address and so on is stored in a contract that inherits from ERC721. The ERC721 mainly tracks ownership, minting and transfer of a token.

One thing you might want to think about is the uniqueness of a token. In your example many tokens can exist that have the same image and description. You might want to prevent that by storing a hash of the _imgHash, _description, _collection and require it to be unique, so that no user can create a "copy" of an existing token.

something like:

keccak256(abi.encodePacked(_imgHash, _description, _collection))

another frequently used standart is the Opensea Metadata Standard, where a tokenURI is stored on the chain and the metadata lives outside the blockchain. https://docs.opensea.io/docs/metadata-standards