Advanced EOS 004 - Table Uniqueness

Blockchain Programming Course
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EOS Multi-Index Tables - Uniqueness

Unique Primary Keys

By now we’re able to create rows in our tables indexed by our unique account_name identifiers, but how do we use a different primary_key and ensure it’s unique? Fortunately the developers at EOSio have provided us with the available_primary_key() function.

Let’s begin by changing our structure’s id property type definition to uint64_t. Remember account_name and name both compile too uint64_t, the only difference is readability.

// @abi table items i64
struct Item {
  uint64_t          id;
  string            name;
  uint64_t          attack;
  account_name      owner;

  auto primary_key() const { return id; };
  EOSLIB_SERIALIZE(Item, (id)(name)(attack));
};

typedef multi_index<N(items), Item> items_table;

Then we will update our create action to generate and set a unique id for each new item we create. Now we will update our create action to use the tables available_primary_key() function like like so;

void inventory::create(const account_name account, const string name, const uint64_t attack) {

  items_table items(_self, _self);

  items.emplace(account, account, [&](auto& item) {
    item.id = items.available_primary_key();
    item.name = name;
    item.attack = attack;
    item.owner = account;
  });
}

Now we’re able to lookup each Item by it’s unique id index. The complete contract code can be found on GitHub.