Minting dependent on NFT ownership

Question

How can I add some additional vaidation to my mint function to check if the person minting the nft owns an item from a specific NFT collection.

In other words, I own 2 collections. I want a user to only be able to mint from collection A in they already own an item in collection B.

function _mint(address to, uint256 tokenId) internal virtual {
    require(to != address(0), "ERC721: mint to the zero address");
    require(!_exists(tokenId), "ERC721: token already minted");

    // some additional check here? 
    // psuedocode: _hasItemInCollection(to);

    _beforeTokenTransfer(address(0), to, tokenId);

    _balances[to] += 1;
    _owners[tokenId] = to;

    emit Transfer(address(0), to, tokenId);
}

Edited: I moved this logic to live in the mint function, not the internal _mint function. Here is the updated code:

function mint(uint256 _mintAmount) public payable mintCompliance(_mintAmount) {
    require(!paused, "The contract is paused!");

    // not A *REAL* contract address, your contract address HERE. 
    address patronAccessToken = 0x000000001234567890ABCDEF0001112022222222;

    bool isPatron = (IToken(patronAccessToken).balanceOf(msg.sender) > 0);
    if (isPatron == true) {
        setCost(0.015 ether);
    }
    require(msg.value >= cost * _mintAmount, "Insufficient funds!");

    _mintLoop(msg.sender, _mintAmount);
}

Thank you for your wisdom!

Answer 1

I'm assuming that both the contracts are created using this template based on the code you posted.

Let's say contract A is deployed at ADDRESS_A. We'll need to add an interface to interact with contract A.

interface IToken {
    function balanceOf(address) external view returns (uint256);
}

Now in our _mint function, we can check if the msg.sender has previously minted in contract A. To do that, we will create an instance of the contract using our interface.

function _mint(address to, uint256 tokenId) internal virtual {
    require(to != address(0), "ERC721: mint to the zero address");
    require(!_exists(tokenId), "ERC721: token already minted");

    require(IToken(ADDRESS_A).balanceOf(msg.sender),"HAVE NOT MINTED A TOKENS")

    _beforeTokenTransfer(address(0), to, tokenId);

    _balances[to] += 1;
    _owners[tokenId] = to;

    emit Transfer(address(0), to, tokenId);
}

Now the mint function will only proceed if the msg.sender balance is non-zero in contract A.