How to access content of mapping in a struct?

Question

I'm aware of this post:

Accessing a mapping within a struct via Truffle console

However, it seems to me, it does not provide a clear answer. Also, my question is more generic and not just for truffle. Also, there's similar code in here (section "structs") but couldn't find any answer to the below questions.

---

Case 1:

Assume we have the following pseudo-code in solidity:

mapping (uint => BB) public map_1; 

struct BB{

  mapping (uint => address) map_2;
}

Question : Given a key, how can I access the content of map_2 in Geth or a user interface?

---

Case 2:

Assume we have the following pseudo-code in solidity:

mapping (uint => BB) public map_1; 

struct AA{
uint val;
}

struct BB{

  mapping (uint => AA) map_2;
}

My question for this case remains the same as the one for case 1.

Answer 1

You can access the case 1s mapping by given the both keys. The key for map_1 and key for map_2. Use pragma solidity ^0.4.20; when compiling

Case 1

contract test {
    struct BB {
        mapping(uint => string) map_2;
    }
    mapping(uint => BB) map_1;

    function test() public {
        map_1[1].map_2[1] = "hello"; //setting temp value
    }

    function maping() public view returns(string) {
        return map_1[1].map_2[1]; //accessing the value
    }
}

Case 2

contract test {
    struct BB {
        mapping(uint => AA) map_2;
    }
    struct AA{
        uint val;
    }
    mapping(uint => BB) map_1;

    function test() public {
        map_1[1].map_2[1].val = 10;
    }

    function maping() public view returns(uint) {
        return map_1[1].map_2[1].val;
    }
}

Answer 2

Case 1

You cannot make map_1 public because the compiler doesn't know how to construct the "free" getter than comes with public. You have to make it yourself, like this:

pragma solidity 0.5.1;

contract Case1 {

    struct BB{
        mapping (uint => address) map_2;
    }

    mapping (uint => BB) map_1; 

    function getter(uint structId, uint mapId) public view returns(address) {
        return map_1[structId].map_2[mapId];
    }  
}

That means you have a function that accepts two uint arguments and returns an address at the requested coordinates.

Case 2

Same as above.

contract Case2 {

    struct AA{
        uint val;
    }

    struct BB{

        mapping (uint => AA) map_2;
    }

    mapping (uint => BB) map_1; 

    function getter(uint BBId, uint AAId) public view returns(uint val) {
        return map_1[BBId].map_2[AAId].val;
    }
}

In this case val in the returns phrase is merely an optional label for the uint that will be returned.

Hope it helps.