When an NFT from one collection gets transfered, transfer certain NFTs from other collection automatically

Question

There're 2 future NFT collections.

Is it possible to create the contracts for them in such a way that whenever an NFT of collection_1 c1 gets, some NFTs from collection_2 c2 of the same account get transfered automatically to the receiver as well?

Meaning, even when a user decides to send an NFT from c1 from his wallet manually, without calling a special function myTransferNFTOf2Collections(...) which I may create. In other words, it should occur in all possible cases, without any additional effort from the user.

option #1:

the contract for collection_1 has been created in the past, and therefore can't be changed now. Would this be possible still?

Answer 1

It is possible to create NFTs bound to other NFTs. Let's say you have an original collection A and a "mirrored" collection B.

By borrowing the ownerOf function from A, we can make one in B like this:

function ownerOf(
    uint256 tokenId
) public view virtual override returns (address) {
    return IERC721(originalCollection).ownerOf(tokenId);
}

you've effectively bound all new NFTs to the owners of the original collection. This is how Cyberkongz have "airdropped" a copy of the original version with on-chain metadata to every holder. When an original is transferred, the internal mapping update happens in one smart-contract, but it is reflected in both. You don't have to use any expensive storage at all which makes it very cheap to deploy.

Note that you may have to fire a specific "batch mint" event when the new contract is deployed in order for off-chain indexers to display the NFTs in user wallets. You can read more about it in a blog by 0xInuarashi.