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in Blockchain by (12.8k points)

I am pretty new to Solidity and working with Ethereum in general. In the (d)app I'm working on I need to be able to persist data onto the ledger, but I'm not sure I understand how this works. Let's say I have the following contract (simplified for practicality):

contract UserContract {

    struct User {

        address walletAddress;

        string organisation;

        string fName;

        string lName;

        string email;

        uint index;

    }

    mapping(address => User) private users;

    address[] private userIndex;

  function insertUser(

        address walletAddress,

        string organisation,

        string fName,

        string lName,

        string email        )

      public

      returns(uint index) {

          User memory newUser = User({

            walletAddress: walletAddress,

            organisation: organisation,

            fName: fName,

            lName: lName,

            email: email,

            index: users.length

            });

            users.push(newUser);

            userIndex[walletAddress] = newUser.index;

            return newUser.index;

    }

}

Using the insertUser() method, I can insert a new user, and using a getter method I could retrieve the user's information.

Now, if I update the contract (thus deploy a new one), the user mapping is empty again, not surprising.

My question: how do I store data in a way that it will be accessible for future versions of the contract? Any design patterns that go along with this process?

Thanks!

1 Answer

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by (29.8k points)

Well, the stored data will not travel with a new contract and copying over data to a new contract would be an expensive task for a bigger user base. In my opinion, your best bet is to separate functionality from data storage.

Create a contract with some basic setters and getters that only deals with data storage (from a particular contract address if necessary), then create your main functional contract that connects to the data contract.

hope this helps

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