Migration contract example

contracts/communication/MessageTributeMigrated.sol

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+pragma solidity >=0.5.0 <0.6.0;
+import "./MessageTribute.sol";
+
+/**
+ * @notice Defines migration contract of MessageTribute
+ */
+contract MessageTributeMigrated is MessageTribute {
+    mapping(address => bool) private migrated;
+    MessageTribute public previousVersion;
+
+    /**
+     * @notice Defines the previous contract to load unset users
+     * @param _previousVersion Contract to read unset tributes
+     */
+    constructor(MessageTribute _previousVersion) public {
+        require(address(_previousVersion) != address(0), "Previous version not provided");
+        previousVersion = _previousVersion;
+    }
+    /**
+     * @notice Obtain required tribute to talk with `_of` from `previousVersion` if `_of` never set on this one
+     * @param _of Account to lookup
+     * @return value of tribute
+     */
+    function getTribute(address _of) external view
+        returns (uint256)
+    {
+        return migrated[_of] ? tributeCatalog[_of] : previousVersion.getTribute(_of);
+    }
+
+    function setTribute(address _of, uint256 _value) internal {
+        migrated[_of] = true;
+        super.setTribute(_of, _value);
+    }
+
+
+}