How-To Check a Record in a Key-Value Table

Overview

This how-to provides instructions to check if a specific object exists in a Key-Value Table (kv table).

Use the method exists defined by the eosio::kv::table::index class to accomplish this task.

Before you begin

Make sure you have the following prerequisites in place:

• An EOSIO development environment, for details consult the Get Started Guide
• A smart contract named smrtcontract
• A user defined type, struct or class, which defines the data stored in the map, named person
• A kv table data type, struct or class, which inherits eosio::kv::table, and stores objects of type person, named address_table

• Each person object has the following data members:

  • account_name,
  • first_name,
  • last_name,
  • personal_id.
• A unique index, named account_name_uidx, defined on the account_name data member

Refer to the following reference implementation for your starting point:

smartcontract.hpp file

struct person {
  eosio::name account_name;
  std::string first_name;
  std::string last_name;
  std::string personal_id;
};

class [[eosio::contract]] smrtcontract : public contract {
    struct [[eosio::table]] address_table : eosio::kv::table<person, "kvaddrbook"_n> {

     index<name> account_name_uidx {
        name{"accname"_n},
        &person::account_name };

     address_table(name contract_name) {
        init(contract_name, account_name_uidx)
     }
  };
  public:
     using contract::contract;
};

Procedure

Complete the following steps to implement an action that is verifying whether a particular person identified by its account_name exists in the address_table:

1. Create a new action verify, which takes as an input parameter the account name of the person to be verified.
2. In the verify action access the instance of address_table by declaring a local variable of address_table type.
3. Call the exists() method of the account_name index defined in the kv table class and pass the account name of the person to be verified.

Refer to the following reference implementation to implement an action that is verifying whether a particular person identified by its account_name exists in the address_table:

smartcontract.hpp

class [[eosio::contract]] smrtcontract : public contract {
    struct [[eosio::table]] address_table : eosio::kv::table<person, "kvaddrbook"_n> {

     index<name> account_name_uidx {
        name{"accname"_n},
        &person::account_name };

     address_table(name contract_name) {
        init(contract_name, account_name_uidx)
     }
  };
  public:
     using contract::contract;

     // checks if a person with a specific account name exists in addressbook
     [[eosio::action]]
     bool verify(eosio::name account_name);

     using verify_action = action_wrapper<"verify"_n, &smrtcontract::verify>;
};

smartcontract.cpp file

// checks if a person with a specific account name exists in addressbook
[[eosio::action]]
bool smrtcontract::verify(string personal_id, string country) {
  address_table addresses{"kvaddrbook"_n};

  return addresses.account_name_uidx.exists({personal_id, country});
}

Summary

In conclusion, the above instructions show how to check if a specific object exists in a Key-Value Table (kv table).

Next Steps

• Implement business logic and rely on the information that the person object exists in the table.