How to loop over soft deleted items on contract?

Question

I have a smart contract that contains posts. Let's say that it's a twitter-like dapp.

Contract Blog {
    struct Post {
        string content;
        bool isDeleted;
    }
    mapping (uint256 => Post) private posts;
    uint256 postCounter = 0;
}

In this contract, I have a get method that returns post if post is not deleted.

function get(uint256 index) public view returns (string memory) {
    Post memory post = posts[index];
    required(post.isDeleted == false, "This post is deleted.")
    return post.text
}

In this case, when I need to loop over Tweets I'm something like this on the frontend.

Get num of posts from the contract = X
for (let i=0; i<=X; i++) {
   try {
       post = try to get post from contract.
   } except {
       if contract returns error suppress it
       continue
   }
}

Most of the time the frontend side is trying to consume the contract for non-existing posts. Is there any better approach to do this?

Does this answer your question? Are there well-solved and simple storage patterns for Solidity? — Ismael, May 05 '21 at 05:21
Using an iteratable mapping may work for you. Also I'd suggest to only use requires in views when the condition will cause a revert. — Ismael, May 05 '21 at 05:24

score 0 · Answer 1 · answered May 05 '21 at 18:32

I have solved this by returning deleted posts with a flag.

function get(uint256 index) public view returns (bool, string memory) {
    Post memory post = posts[index];
    return (post.deleted, post.text)
}

Then after on frontend side I'm using that deleted flag to not to show.

Get num of posts from the contract = X
for (let i=0; i<=X; i++) {
    isDeleted, text = try to get post from contract.
    if (isDeleted) {
        continue (or show an empty post that describes that is deleted)
    } else {
        render post
    }
}

How to loop over soft deleted items on contract?

1 Answers1