corda/docs/source/tut-two-party-contract.rst

Updating the contract

Remember that each state references a contract. The contract imposes constraints on transactions involving that state. If the transaction does not obey the constraints of all the contracts of all its states, it cannot become a valid ledger update.

We need to modify our contract so that the lender's signature is required in any IOU creation transaction. This will only require changing a single line of code. In IOUContract.java/IOUContract.kt, update the final line of the requireThat block as follows:

"The borrower and lender must be signers." using (command.signers.containsAll(listOf(
    out.borrower.owningKey, out.lender.owningKey)))

check.using("The borrower and lender must be signers.", command.getSigners().containsAll(
    ImmutableList.of(borrower.getOwningKey(), lender.getOwningKey())));

Progress so far

Our contract now imposes an additional constraint - the lender must also sign an IOU creation transaction. Next, we need to update IOUFlow so that it actually gathers the counterparty's signature as part of the flow.