Ethers.js Mapping Struct – Retrieve a Mapping Nested Inside a Struct from Ethers

ethers.jsmappingstruct

Hello Ethereum community,

I have couple of structs in my solidity code to organize things. Here is one struct from the code:

struct Staking {
  uint total;
  uint currentIndex;
  mapping(address => uint) registry;
}

Staking     public staking;

When compiling, I also get a staking() getter in the abi. Now on my frontend I have configured ethers npm library and am trying to retrive all of my structs. Calling

const data = await ethers.gameContract.staking();

Gives me this, JSON.stringified

[{"type":"BigNumber","hex":"0x00"},{"type":"BigNumber","hex":"0x00"}]

I can confirm that those two are total and currentIndex uints. But I can't see registry mapping.

Is there a way to query data modeled like this, or should I just declare all these properties in the top level of contract block (which should solve the issue by creating a getter for each prop in the abi), or maybe create custom getter functions that will retrieve these nested mappings?

EDIT: Calling staking() from the Remix in VSCode, I get this data:

{"0":"0","1":"0","total":"0","lastIndex":"0"}

Now I'm definitely confused. Are the first 2 items in this object, "0" and "1" keys actually items from the mapping? Maybe I am asking a wrong question here. But I still can't figure out how to query this mapping.

Thanks!

Best Answer

The ABI doesn't contain them due to their dynamic size. The accessory of maps and arrays does not return the whole data; hence, we query according to our needs.

Well for mapping, we don't have any way to iterate it. This works quite well for declaring state variables, but it is not possible to supply the argument for a struct with an array or map (dynamic) as a member.

Also, the struct is turned into a tuple of all its elementary members and the Non-elementary members are stripped from the tuple.

Related Solutions

Solidity Memory Management – Does ‘Delete’ Free Mapping Space

Based on my Debug observation:

I have tested that:

Firstly I insert data into my map data-structure as I explained in my question.
Secondly, I have returned the address of first mapped index, which is 0, this could be done for other indexes as well (0:10).
Without a delete operation I can retrieve the data perfectly fine from the address of mapped Foo[]. But if I did the delete operation, Solidity gives an error.

This debug result: leads me to conclude that memory is freed or address does not contain any valid information.

Test code:

set_txn_hash     = my_contract.transact().map_insert();
contract_address = unmigrated_chain.wait.for_receipt(set_txn_hash)

for x in range(0, 9):
   for y in range(0, 9):
         output           = my_contract.call().get_(x, y);
         print(output);  //returns correct value.

set_txn_hash     = my_contract.transact().map_remove_map();                                             
contract_address = unmigrated_chain.wait.for_receipt(set_txn_hash)                                      

set_txn_hash     = my_contract.transact().get_map_address(0);
contract_address = unmigrated_chain.wait.for_receipt(set_txn_hash)

output           = my_contract.call().get_map();
print(output);

output           = my_contract.call().try_();
print(output);  //Solidity gives error if delete map_remove_map() is called

Additional functions inside my Contract:

  function map_insert(){
    for(uint i = 0; i < 10; i++)
      for(uint j = 0; j < 10; j++)
        add(i, i*10 + j);
  }

  function map_remove_map(){
    for(uint i = 0; i < 10; i++)
      delete_(i);
  }

  function map_remove_map_item(){
    for(uint i = 0; i < 10; i++)
      for(uint j = 0; j < 10; j++)
        delete_items(i, j);
  }

  function get_map_address(uint id) {
    uint addr;
    Foo[] a = foo[id];

    assembly{
    addr := a
        }
    map_address = addr;
  }

  function get_map() constant returns (uint){
    return map_address;
  }

  function try_() constant returns (uint ){
    uint addr = map_address;
    Foo[] a;
    assembly{
    a := addr
        }
    return a[1].x;  
  }

Solidity Structs – Accessing Mapping Inside Mapping of Struct in Solidity ^0.5.0

You can't do with a getter from public because the compiler doesn't (yet) know how to formulate the function with nested keys. I believe this is possible if you specific you wish to the experimental ABI.

A safer, more conventional workaround is to simple construct your own getter.

contract a { 

  struct Str {
    uint256 totalTokens;
    mapping(address => uint256) playerTokens;
  }

  mapping(uint256 => Str) private tokenStores;  // we'll make the getter manually

  function getPlayerToken(uint256 tokenId, address player) public returns(uint, uint) {
      Str storage t =  tokenStores[tokenId];
      return (t.totalTokens, t.playerTokens[player]);
  }
}

Hope it helps.

Related Topic

Solidity Mapping – How to Access Nested Struct Mapping Members