How can we retrieve table data with secondary Index in EOS table

Question

this is my table

  struct [[eosio::table]] payment_made
  {
    uint64_t pay_id;
    uint64_t loan_id;
    uint64_t pay_amunt;

    time_point_sec pay_date_req;
    time_point_sec pay_date_approve;

    bool payment_status= false;
    // name pay_from;

    uint64_t primary_key() const
    {
      return pay_id;
    }

     uint64_t by_loanId() const
    {
      return loan_id;
    }
  };
 typedef eosio::multi_index<"paymentmade"_n, payment_made,    
                       indexed_by<"loanid"_n, const_mem_fun<payment_made, uint64_t, &payment_made::by_loanId>>>paymentmade;


 paymentmade paymade_table;

Here my loan_id is Secondary multi-indexed key

Here i want to modify my table,for that i will need the private key of the table so,i want to get the private key of table with the help of secondary key please help?

I want to modify
pay_date_approve and payment_status in this function and the only argument is getting passed is loan_id which secondary key of the table,so can we do this ?

void mainloan::payconfirm(uint64_t loan_id)
{

 auto pridx = paymade_table.get_index<N(loanid)>();
}

Answer 1

Do same like primary key index:

void mainloan::payconfirm(uint64_t loan_id) {
  // assume paymade_table is instantiated through contract constructor
  auto pridx = paymade_table.get_index<N(loanid)>();
  auto itr = pridx.find( SECONDARY_KEY_WHICH_YOU_WANT_TO_FIND );
  pridx.modify(itr, same_payer, [&](auto& p) {
    /* updater */
  });
}