Mapping 1 to many ownership

Question

this is probably a really really simple question but shamefully it's had me stumped for a while. I am building a simple application which would allow users to purchase Newspaper Articles. Simple enough but the relations is killing me.

I'll outline my goal and where I am so you get an idea of my thinking and perhaps can point me in a more logical direction if needed.

So the way I see it, I have 3 entities.

An Author, An Article and A Reader

An Author can have many Articles

An Article can have one Author

An Article can have many Readers

A Reader can have many Articles

An Author and a Reader have no relation

Mapping a new Article to an Author is not a problem and I have this working

My problem is with mapping many Articles with many Readers

I've thought about having a mapping table like ArticlesRead which would simply have the address of each Reader & the ID of each Article, but how do I go about retrieving the Articles the Reader has purchased.

I think I'm getting a little hung up on how traditional arrays work..

I'm sure this is simple so any advice would be appreciated,

struct Article {
    uint index;
    string content;
    uint price;
}

struct Reader {
    address _address;
}

My code is pretty rubbish for now so I don't think it will help posting it

Thanks

Answer 1

This is not that simple, do not worry :) indeed you need to map to a structure itself containing arrays.

corresponding to the following code

struct Article {
    uint index;
    string content;
    uint price;
    address[] readerId;
}

struct Reader {
    address[] articleId;
}

struct Author {
    address[] articleId;
}

// map id to id
mapping(address => address) mapArticleIdToAuthorId; //only valid if an article always has one and only one Author

// map id to Struct
mapping(address => Article) mapArticleIdToArticleStruct;
mapping(address => Author) mapAuthorIdToAuthorStruct;
mapping(address => Reader) mapReaderIdToReaderStruct

instead of address you can of course use other types (like uint256).

I am looking forward seing if anyone is coming with some alternative solution !

Hope this helps

extra: I have some code samples (with uint, I let you adapt them for address type) for adding / removing items

you can add entry in a array as such

// add articleId to author struct
Author storage author = mapAuthorIdToAuthorStruct[_authorId];
author.articleId.push(_articleId)

and removing an entry:

// remove articleId from author struct
Author storage author = mapArticleIdToAuthorStruct[_articleId];
uint[] memory temp = new uint[](author.articleId.length-1);
uint j = 0;
for (uint i = 0; i < author.articleId.length; i++) {
    if (author.articleId[i] != _articleId) {
        temp[j] = author.articleId[i];
        j++;
    }
}
author.articleId = temp;
// remove mapping between articleId and authorId
mapArticleIdToAuthorId[articleId] = 0;

keep in mind that storage is expensive in ethereum. Gas costs are defined in the yellow paper