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 borrower’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 two lines of the requireThat block as follows:

// Constraints on the signers.
"There must be two signers." using (command.signers.toSet().size == 2)
"The borrower and lender must be signers." using (command.signers.containsAll(listOf(
    out.borrower.owningKey, out.lender.owningKey)))

...

import com.google.common.collect.ImmutableList;

...

// Constraints on the signers.
check.using("There must be two signers.", command.getSigners().size() == 2);
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 borrower must also sign an IOU creation transaction. Next, we need to update IOUFlow so that it actually gathers the borrower’s signature as part of the flow.