I'm trying to define a mapping, where each value should be an array of struct. Not sure, if this is possible? If yes then what wrong I'm doing here?

After defining, I also need to push values to the array as shown below. Please advise if it is correct.

struct Review {
    uint rating;
    address reviewer;
    string description;
    uint dateAdded;

mapping (uint => Review[]) public ratings;

function rateBook(uint id, uint rating, string comments) onlyMember {
    if (id > numBooks || rating < 1 || rating > 5) {

        rating: rating,
        reviewer: msg.sender,
        description: comments,
        dateAdded: now

1 Answer 1


Yes this is correct, here's my test code:

import "dapple/test.sol";

contract MappingArrayOfStructs is Test {

  struct Review {
    uint rating;
    address reviewer;
    string description;
    uint dateAdded;

  mapping (uint => Review[]) ratings;

  function testMappingArrayOfStructs() {
    //@log push to ratings array
      rating: 0,
      reviewer: msg.sender,
      description: "",
      dateAdded: now
    //@log `uint ratings[0][0].dateAdded`


This can be run with dapple test --report and produces the following report:

  test mapping array of structs
  LOG:  push to ratings array
  LOG:  1493284381

When you make a state variable public, the compiler automatically generates a getter function as described in the documentation https://solidity.readthedocs.io/en/latest/contracts.html#getter-functions:

function ratings(uint arg1, uint arg2) returns (uint rating, address reviewer, string description, uint dateAdded) {
    rating = ratings[arg1][arg2].rating;
    reviewer = ratings[arg1][arg2].reviewer;
    description = ratings[arg1][arg2].description;
    dateAdded = ratings[arg1][arg2].dateAdded;

You can access the member data in Solidity like this: var (rating, reviewer, description, dateAdded) = reviews.ratings(0, 0);

If you are using Truffle then the generated bindings treat tuples as arrays, so you can access the member data in JS like this:

var Reviews = artifacts.require("./Reviews.sol");

contract('Reviews', function(accounts) {
  it("should return the correct book rating", function() {
    var reviews;
    return Reviews.deployed().then(function(instance) {
      reviews = instance;
      return reviews.rateBook(0, 5, "This book changed my life");
    }).then(function() {
      return reviews.ratings.call(0, 0);
    }).then(function(review) {
      assert.equal(review[0], 5, "5 wasn't the rating for the book with id 0");

Likewise, a Truffle themed test would work like this:

pragma solidity ^0.4.2;

import "truffle/Assert.sol";
import "truffle/DeployedAddresses.sol";
import "../contracts/Reviews.sol";

contract TestReviews {

  function testRateBookUsingDeployedContract() {
    Reviews reviews = Reviews(DeployedAddresses.Reviews());

    reviews.rateBook(0, 5, "Test comment");
    var (rating, reviewer, description, dateAdded) = reviews.ratings(0, 0);

    Assert.equal(rating, 5, "Book should have a rating of 5");


On the command line you can simply run these tests using truffle test and you will get the following output:

Compiling ./contracts/Migrations.sol...
Compiling ./contracts/Reviews.sol...
Compiling ./test/TestReviews.sol...
Compiling truffle/Assert.sol...
Compiling truffle/DeployedAddresses.sol...

    ✓ testRateBookUsingDeployedContract (38ms)

  Contract: Reviews
    ✓ should return the correct book rating (65ms)

  2 passing (198ms)
  • One more question on this, how do I access ratings outside of contract? I'm writing a test case in JS and this is how I was trying to but every time it is coming as undefined. // lms is the constructor of my contract let rating = await lms.ratings[1][accounts[0]].rating; //code fails here itself let description = await lms.ratings[1][accounts[0]].description; console.log(rating, description);
    – Sanchit
    Apr 27, 2017 at 10:19
  • I've updated the answer with the information. Apr 27, 2017 at 11:30
  • Awesome! Thank you so much for explaining this. You are the Man!
    – Sanchit
    Apr 27, 2017 at 19:11

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