Cannot call functions from another deployed contract that inherits yet another contract

Question

I am just trying this using Remix editor. I am able to call functions from another deployed contract(plain as simple) but when that another deployed contract inherits yet another contract, Now the calling functions from the original contract to the deployed contract doesn't work. I just get a transaction revert

Working Code:

pragma solidity >=0.4.22 <0.6.0;
contract Existing {
    Deployed public dc;
    constructor(address _deployedContractAddress) public {
        dc = Deployed(_deployedContractAddress);
    }

    function setA(uint _a) public returns (uint) {
        return dc.setA(_a);
    }
    function getA() public view returns (uint) {
        return dc.a();
    }
}

contract Deployed {
    uint public a = 1;

    function setA(uint _a) public returns (uint) {
        a = _a;
        return a;
    }

}

Not Working Code:

pragma solidity >=0.4.22 <0.6.0;
contract Existing {
    Deployed public dc;
    constructor(address _deployedContractAddress) public {
        dc = Deployed(_deployedContractAddress);
    }

    function setA(uint _a) public returns (uint) {
        return dc.setA(_a);
    }
    function getA() public view returns (uint) {
        return dc.a();
    }
}


contract Ownable {
  address public owner;

  constructor() public {
    owner = msg.sender;
  }

  /**
   * @dev Throws if called by any account other than the owner.
   */
  modifier onlyOwner() {
    require(msg.sender == owner);
    _;
  }

}

contract Deployed is Ownable {
    uint public a = 1;

    function setA(uint _a) public onlyOwner returns (uint) {
        a = _a;
        return a;
    }

}

So here, the contract Deployed is inheriting from contract Ownable and I just get a transaction revert on calling setA

Note: contract Deployed is seperately deployed in a separate file and also included in contract Existing file.

Answer 1

The problem is not in the inheritance but in the logic of which address owns which contract. In the second (non-working) example if you remove the onlyOwner modifier from the function it works just fine.

In the non-working example this is what happens:

1) You deploy contract Deployed from your wallet, let's say its address is 0x123. That wallet becomes the owner of Deployed (as per the contract Ownable).

2) You deploy contract Existing from the same wallet. No ownership is present here.

3) Contract Existing uses the existing instance of Deployed for making a call to it. Here msg.sender is 0x123.

4) When the call from Existing to Deployed reaches Deployed the msg.sender is the originating contract's address - so the address of Existing. That contract is not the owner of Deployed as the owner is 0x123. Therefore, as the function requires that only the owner can call it, the call fails and the whole transaction is reverted.