Updating a struct property in an array stored as mapping value

Question

This is my data structure for storing a list of questions which will be different for every address

struct Question{
    uint16 id;
    string question_string;
    string answer;
}

and this is stored as an array being the value of the mapping while address of the user being the key

mapping(address=>Question[]) public user_questions;

Now I want to update a particular question's answer for a user. What can I do for the same?

I tried using the following function:

function addAnswer(address _user, uint16 _id, string memory _answer) public {
    Question[] storage ques = user_questions[_user];
    ques[_id].answer = _answer;
}

but I get an error saying

[vm] from: 0x5B3...eddC4 to: AnonAMA.addAnswer(address,uint16,string) 0xdda...5482d value: 0 weight data: 0x0ed...00000 logs: 0 hash: 0xf7c...fdf84

transact to AnonAMA.addAnswer errored: VM error: revert.

revert The transaction has been reverted to the initial state.

Note: The called function should be payable if you send value and the value you send should be less than your current balance. Debug the transaction to get more information.

Answer 1

It looks like you should add Question struct into the mapping array first, then you can update the _answer field.

I am not sure the Question was added before into the array mapping, but if it was not the code would look something like this:

function addQuestion(address _user, Question storage question) public returns (uint16) {
    user_questions[_user].push(question);

    // Returns Question array index (_id)
    return user_questions[_user].length - 1;
}

Once you have called this, the function that you have written addAnswer() should work - assuming _id is valid

Answer 2

Question[] is an array, not a mapping. You must push a Question into this array before you can set values on it. If you must dynamically set values without pushing into the array first, you must use a nested mapping.

mapping(address => mapping(uint16 => Question)) public user_questions;

Where the uint16 is your Question id. This eliminates the need for the array and allows for you to update values without having to initialize their array index.