How to upgrade a multi_index table in a smart contract

Question

I have initially the following multi_index table:

struct [[eosio::table]] plastict
{
    uint64_t id;
    uint64_t type;

    uint64_t primary_key() const { return id; }
};
typedef eosio::multi_index<"items"_n, item> items;
items items_table;

I have deployed initial smart contract and now am I trying to add a new field to the the table items. According to the info, I have found in a couple of EOS issues, I have added the new field at the end of the table struct:

struct [[eosio::table]] plastict
{
    uint64_t id;
    uint64_t type;
    uint64_t state;

    uint64_t primary_key() const { return id; }
};
typedef eosio::multi_index<"items"_n, item> items;
items items_table;

Unfortunately, after I have re-compiled and re-deployed the smart contract, when I try to read the table content, I get the following:

Error: {"code":500,"message":"Internal Service Error","error":{"code":3015013,"name":"unpack_exception","what":"Unpack data exception","details":[{"message":"Stream unexpectedly ended; unable to unpack field 'state' of struct 'item'","file":"abi_serializer.cpp","line_number":307,"method":"_binary_to_variant"}]}}

I don't need backward compatibility, thats why I am not wrapping the new field within binary_extension type, but I want to keep all the data already inserted and keep it accessible.

Appreciate any ideas and remarks ?

Answer 1

If you don't care about the data from old_table:

• Erase all records from old_table
• Deploy NEWCONTRACT with modified old_table

---

If you want to persist the data from old_table:

• Define the new and old structs/tables in NEW_CONTRACT1
• Write a function that will move data from old_table to new_table; adding new column
• Deploy NEW_CONTRACT2 version that has old_table modified the way you want
• Reverse the data migration function to move data from new_table back to old_table

Now your user will be able to query old_table the way they did before and the contract will not throw an exception. You simply cannot modify a table that has any existing data inside of it.

I have an example contract for this process you can view at: https://github.com/netuoso/eos-migrate-demo

Note: If you have a table with many records, it is possible the transaction time limit will be reached. In that situation, you could modify the migrate function to only move a limited number of records each time you call it.