mapping_mapping

Questions.sol

// SPDX-License-Identifier: MIT

pragma solidity ^0.6.6;

contract Questions {
    uint256 public questionIdCounter;
    mapping(uint256 => Question) public questions;

    struct Question {
        uint256 answerCounter;
        mapping(uint256 => Answer) answers;
    }

    struct Answer {
        address answerer;
    }

    constructor() public{
        questionIdCounter = 0;
    }

    function askQuestion() public {
        questions[questionIdCounter] = Question({answerCounter: 0});
        questionIdCounter = questionIdCounter + 1;
    }

    function answerQuestion(uint256 questionId) public {
        // Add new answer
        questions[questionId].answers[questions[questionId].answerCounter] = Answer({answerer: msg.sender});
        // update answer counter
        questions[questionId].answerCounter = questions[questionId].answerCounter + 1;
    }
    
    function getAnswer(uint256 questionId, uint256 answerId) public returns (address){
        return questions[questionId].answers[answerId].answerer;
    }
    
}