How to interact with mapping of mappings for structs in solidity? - ethereum

Question

I have created struct with nested mapping. Consider the following piece of code written inside the contract in solidity:
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.4;
contract TestContract {
struct Item {
uint itemId;
uint itemValue;
}
struct OverlayingItemStruct {
mapping(uint => Item) items;
uint overlayingId;
uint itemsCount;
}
mapping(uint => OverlayingItemStruct) public overlayingItems;
uint public overlayCount;
function addOverlayingItemsStruct(uint _value) external {
overlayCount ++;
mapping(uint => Item) memory items;
items[1] = Item(1, _value);
overlayingItems[overlayCount] = OverlayingItemStruct(items, 1, 1);
}
function addItem(uint _value, uint _overlayId) external {
OverlayingItemStruct storage overlay = overlayingItems[_overlayId];
overlay.itemsCount ++;
overlay.items[overlay.itemsCount] = Item(overlay.itemsCount, _value);
}
}
When compiling the above code, I am getting the error:
TypeError: Uninitialized mapping. Mappings cannot be created dynamically, you have to assign them from a state variable.
--> TestC.sol:21:5:
|
21 | mapping(uint => Item) memory items;
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Not sure, how to handle this error.
Secondly, needed to confirm if the method to add new values in the nested mapping dictionary is correct?

Answer 1

A mapping cannot be declare inside a function and in memory state. This type has only storage state.
Said this, to pass a mapping into a struct, you can set the key of nested mapping and pass relative struct inside it.
I tried to adjust your smart contract like this:
// SPDX-License-Identifier: MIT
pragma solidity ^0.8.4;
contract TestContract {
struct Item {
uint itemId;
uint itemValue;
}
struct OverlayingItemStruct {
mapping(uint => Item) items;
uint overlayingId;
uint itemsCount;
}
mapping(uint => OverlayingItemStruct) public overlayingItems;
uint public overlayCount;
function addOverlayingItemsStruct(uint _value) external {
overlayCount ++;
// NOTE: I declared a new Item struct
Item memory item = Item(1, _value);
// NOTE: I set into items mapping key value 1, Item struct created in row above this
overlayingItems[overlayCount].items[1] = item;
}
function addItem(uint _value, uint _overlayId) external {
OverlayingItemStruct storage overlay = overlayingItems[_overlayId];
overlay.itemsCount ++;
overlay.items[overlay.itemsCount] = Item(overlay.itemsCount, _value);
}
}