How can a smart contract payable based function returns value against any purchase

Question

I am trying to get a return value from the payable function, the idea is a user must pay the fees and in return, he will get his special code. My JS test code is as follow

it('get special code which was stored by the admin',async()=> {

        const user = accounts[4];

        // get the deployed contract instance
        contractInstance = await MyContract.deployed()

        const expectedCode = "Wolvorine";

        const fees = "1";  

        let result1 = await contractInstance.getCode(user,{from:user,value:fees}) ;

    assert.equal(result1,expectedCode, " Code which was stored by the admin must be equal to the expected value")

My contract which returns the value is as below

 function getCode(address _identityaddress) payable
public returns (string memory)
{
    require((identity_details[_identityaddress].identityaddress==msg.sender));

    require(msg.value >= attestationFee);

    Transaction memory p = identity_details[_identityaddress];

    return (p.code);
}

Here Transaction is a structure data type that has code as a string variable inside. If I make this function without payable and don't take any money from the caller then return value accurately returns me the code. But since I am using payable and receiving ethers, therefore, it is returning me a big transaction object instead of my returned value.

What is the best way to tackle this issue since I want the user to use MetaMask only one time where he will just pay and then get this value in one shot?

Answer 1

If I make this function without payable and don't take any money from the caller then return value accurately returns me the code.

You are on the right track but misinterpreting what you see. I'll break it down.

accurately returns me the code

I think you mean returns(string memory).

without payable

Payable has nothing to do with it. You have to add either view or constant to make it a read-only function as the compiler will suggest you do if you remove payable because without value transfer, this function doesn't change anything. So, I think that's what happened, or something very similar.

This issue relates to Ethereum's treatment of state-changing (write) and read-only operations. In the case of a state-changing function the sender never gets the expected return value, but another contract does. The reason for this is the sent transactions have not been mined, so the return value is unknown.

The receipt you get is an identifier your node produced to let you know how to track the transaction. This callback confirms that your node is aware of your desire to submit something to the network. It's not the same concept as "result".

It's a confusing topic and it implies front-end design patterns that deal with waiting for mining. Have a look over here for an explainer: https://blog.b9lab.com/calls-vs-transactions-in-ethereum-smart-contracts-62d6b17d0bc2

Hope it helps.