Upgradeable ERC721

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Hi @lucianstroie,

OpenZeppelin SDK (previously ZeppelinOS) creates upgradeable contracts.
I recommend using the latest version which is OpenZeppelin SDK 2.6

If you are using OpenZeppelin SDK and upgradeable contracts you need to use @openzeppelin/contracts-ethereum-package rather than @openzeppelin/contracts.

https://docs.openzeppelin.com/sdk/2.5/linking
NOTE: Make sure you install @openzeppelin/contracts-ethereum-package and not the vanilla @openzeppelin/contracts. The latter is set up for general usage, while @openzeppelin/contracts-ethereum-package is tailored for being used with the OpenZeppelin SDK. This means that its contracts are already set up to be upgradeable.

I recently created an issue to add a warning to users when they create a contract importing @openzeppelin/contracts: OpenZeppelin/openzeppelin-sdk#1297 .
There is also a plan to remove this requirement to use the Contracts Ethereum Package version: Planning the demise of OpenZeppelin Contracts’ evil twin.

Also with upgradeable contracts we need to use initializers rather than constructors.

https://docs.openzeppelin.com/sdk/2.5/writing-contracts
You can use your Solidity contracts in the OpenZeppelin SDK without any modifications, except for their constructors. Due to a requirement of the proxy-based upgradeability system, no constructors can be used in upgradeable contracts.

An example upgradeable ERC721 is as follows:

Simple721Token.sol

When the contract is created (openzeppelin create) need to ensure it is initialized passing in the address to have the minter role.

Tokens can then be minted by calling (openzeppelin send-tx) mintWithTokenURI

pragma solidity ^0.5.0;

import "@openzeppelin/upgrades/contracts/Initializable.sol";

import "@openzeppelin/contracts-ethereum-package/contracts/token/ERC721/ERC721Full.sol";
import "@openzeppelin/contracts-ethereum-package/contracts/token/ERC721/ERC721Mintable.sol";

contract Simple721Token is Initializable, ERC721Full, ERC721Mintable {

    function initialize(address sender) public initializer {
        ERC721.initialize();
        ERC721Metadata.initialize("Simple721Token", "721");
        ERC721Enumerable.initialize();
        ERC721Mintable.initialize(sender);
    }
}