Returning a struct and reading via Web3

  • I am storing data in my contract using a mapping of structs.

    For examples sake, say I'm storing employee info (name/address/salary), mapped by their employee ID.

    Via a web front-end, I'd like to be able to specify an employee ID, and call a function in my contract (using Web3.JS) that returns the employee info.

    How can I access the data if a struct is returned. Is that possible?

  • Summary

    Return the fields of the struct as separate return variables.



    Example

    I'm running this code in my local dev blockchain using the following command:

    geth --datadir ~/devdata --dev --nodiscover \
      --mine --minerthreads 1 --port 30301      \
      --maxpeers 0 --verbosity 3 --rpc console
    

    Your web frontend should be able to send transactions to insert the users, and call the functions to get the number of users and and the user info.


    Sample Contract

    contract SalaryInfo {
        struct User {
            uint salaryId;
            string name;
            string userAddress;
            uint salary;
        }
        User[] public users;
    
        function addUser(uint _salaryId, string _name, string _userAddress, uint _salary) public returns(uint) {
            users.length++;
            users[users.length-1].salaryId = _salaryId;
            users[users.length-1].name = _name;
            users[users.length-1].userAddress = _userAddress;
            users[users.length-1].salary = _salary;
            return users.length;
        }
    
        function getUsersCount() public constant returns(uint) {
            return users.length;
        }
    
        function getUser(uint index) public constant returns(uint, string, string, uint) {
            return (users[index].salaryId, users[index].name, users[index].userAddress, users[index].salary);
        }
    }
    


    Flatten The Source Code

    I use the stripCrLf method (from How to load Solidity source file into geth) to transform the formatted source into a single line that can be inserted within the geth console. Alternatively, search for a web page that will strip your line breaks from your code. Then assign your code to a JavaScript variable:

    > var salaryInfoSource='contract SalaryInfo { struct User { uint salaryId; string name; string userAddress; uint salary; } User[] public users; function addUser(uint _salaryId, string _name, string _userAddress, uint _salary) public returns(uint) { users.length++; users[users.length-1].salaryId = _salaryId; users[users.length-1].name = _name; users[users.length-1].userAddress = _userAddress; users[users.length-1].salary = _salary; return users.length; } function getUsersCount() public constant returns(uint) { return users.length; } function getUser(uint index) public constant returns(uint, string, string, uint) { return (users[index].salaryId, users[index].name, users[index].userAddress, users[index].salary); }}'
    


    Insert Contract Into The Blockchain

    The compile the code:

    > var salaryInfoCompiled = web3.eth.compile.solidity(salaryInfoSource);
    

    Load the code into the blockchain:

    > var salaryInfoContract = web3.eth.contract(salaryInfoCompiled.SalaryInfo.info.abiDefinition);
    > var salaryInfo = salaryInfoContract.new({from:web3.eth.accounts[0], data: salaryInfoCompiled.SalaryInfo.code, gas: 1000000}, 
      function(e, contract) {
        if (!e) {
          if(!contract.address) {
            console.log("Contract transaction send: TransactionHash: " + 
              contract.transactionHash + " waiting to be mined...");
          } else {
            console.log("Contract mined! Address: " + contract.address);
            console.log(contract);
          }
        }
      }
    )
    

    Wait for the following message to indicate that the contract has been mined:

    I0505 09:12:15.712867   27030 xeth.go:1026] Tx(0x7747500b881c8da44efbc3b5d1c2c762f1cd52d2dd74050edbfed10e51a29d8a) created: 0x0bb1d7a6b31f7a7e23e6d902bac0eb5f9c721c54
    Contract transaction send: TransactionHash: 0x7747500b881c8da44efbc3b5d1c2c762f1cd52d2dd74050edbfed10e51a29d8a waiting to be mined...
    ...
    Contract mined! Address: 0x0bb1d7a6b31f7a7e23e6d902bac0eb5f9c721c54
    [object Object]
    


    Insert Users

    And here we are adding 2 users to the contract:

    > salaryInfo.addUser(123, "User 123", "123 drive way, the uncentralised kingdom", 100, {from:web3.eth.accounts[0], data: salaryInfoCompiled.SalaryInfo.code, gas: 500000});
    "0x7c22797d6b7717beb398a65159b1009fba3bbc9e4917ee1584bed60ea74eac11"
    > salaryInfo.addUser(234, "User 234", "234 drive way, the uncentralised kingdom", 200, {from:web3.eth.accounts[0], data: salaryInfoCompiled.SalaryInfo.code, gas: 500000});
    "0xed197c9a6fbc70c19cc95bcdc6943e38736c080052e9e1a4f7562216d6de4c78"
    


    Retrieve Data

    Let's get the number of users:

    > var numberOfUsers = salaryInfo.getUsersCount();
    undefined
    > numberOfUsers
    2
    

    Let's get the information for the first user:

    > salaryInfo.getUser(0)
    [123, "User 123", "123 drive way, the uncentralised kingdom", 100]
    

    And the second user:

    > var user2 = salaryInfo.getUser(1);
    undefined
    
    > user2
    [234, "User 234", "234 drive way, the uncentralised kingdom", 200]
    

    Wow! This is incredibly helpful. Thank you so much for your in-depth answer. I really appreciate it!

    I get a BigNumber not a Base 16 number... how can I return string attributes with your kind of solution?? (I'm using web3 0.16)

    @BokkyPooBah same issue. Getting same big nunber issue. Any help on this?

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Content dated before 7/24/2021 11:53 AM