Using find on multi index table

Question

I have a table defined as the following:

struct [[ eosio::table, eosio::contract("voting") ]] identity
{
    uint64_t key;
    key256 identityHash;
    time_point_sec createdAt;

    uint64_t primary_key() const { return key; }
    key256 by_hash() const { return identityHash; }
};
typedef multi_index<"identity"_n, identity,
    indexed_by< "by_hash"_n, const_mem_fun<identity, key256, &identity::by_hash> >
> identity_index;

I now want to use .find on the table using the secondary index but I am unable to do so. I am trying the following:

voting::identity_index identities(_self, _self.value);
auto itr = identities.find(out);
eosio_assert(itr == identities.end(), "User already registered to vote!");

But the above code just searches on the primary key. Is there a way to search on the secondary key?

Answer 1

See https://developers.eos.io/eosio-cpp/docs/using-multi-index-tables for more info

It might not be possible to use .find on the second index, but you could always iterate through each element and compare relevant values. I believe runtime should be the same.

From the above example:

std::vector<uint64_t> keysForDeletionFromVotes;
    // find items which are for the named poll
    for(auto& item : _votes) {
        if (item.pollName == pollName) {
            keysForDeletionFromVotes.push_back(item.key);   
        }
    }

Related answer: Multiple parameters for search

Answer 2

To use the secondary index, you need to use get_index().

voting::identity_index identities(_self, _self.value);
auto idx = identities.get_index<"by_hash"_n>();
auto itr = idx.find(out);
eosio_assert(itr == identities.end(), "User already registered to vote!");