Current functionality:
When a payment is created, you have the option to link it to one or more invoices. The payment-invoice relationship is then set and can not be manually modified by the user.

Desired functionality:
After a payment is created, the user should be able to add/remove links to invoices. From the payment details screen, the user should be able to:
- remove a link to an invoice. The invoice's balance would be increased by the amount that the payment had paid of it. The same thing happens to the payment's balance.
- create a link to an invoice if the payment has any balance left. In that case, the invoice gets paid by the amount left in the payment's balance, just like it is now when a new payment is created.

Technical overview:

Client tier modifications:
- A link 'Pay an invoice with this payment' is displayed only if the payment's balance is != 0. This is to allow an existing payment to be linked to an invoice.
- The list of invoices being paid should be modified to show a link 'Remove' in each row.
- Removal of a link to an invoice prompts a confirmation screen.
- All the above changes apply only to non-customer profiles. In other words, customers can not link/unlink their payments to invoices. :)

Database tier modifications:
- Take a look to the payment database diagram, available in the documentation section of the website. The table that connects the payment and invoice tables is just a map table that holds the ids of the related rows. This table has to be changed by one with an additional column with the amount that the payment has paid of the linked invoice. The name of the table has to change has well, because we reserve the name *_map to tables that are only used to allow for an N to N relationship and therefore only hold two ids. So just replace the name by 'payment_invoice', dropping the map suffix.
- Since now we have a 'new' table, we need a new entity bean to map it as a Java object. This will be an entity bean with CMR that returns one bean for payment and one for invoice. So we'll have something like this for the getters (you can imagine the setters):
Float getAmount();
PaymentEntity getPayment();
InvoiceEntity getInvoice().
- The existing CMR relationship between the payment an invoice tables is being replaced by our new table. So these two entity beans have to be modified.

Server tier modifications:
- Since payment and invoice entity beans have changed, you'll need to update the calls to the changed methods in a few places. Of special interest is PaymentSessionBean.applyPayment(). The compiler will tell you all the places you need to update, you'll have to update them by using the new entity bean.
- Move the code in PaymentSessionBean.applyPayment to PaymentBL, where should have been from the beginning. You still need the method in PaymentSessionBean, but simply forward the call to PaymetBL. Remember, session beans are there only as facades, to expose only some methods to the client tier as for transaction demarcation.
- We need a new method PaymentBL.unapplyPayment. This has to undo what today is done with PaymentSessionBean.applyPayment. This basically is:
- Add to the invoice balance whatever is in payment_invoice.amount
- Add to the payment balance whatever is in payment_invoice.amount
- If the invoice balance is >= 0.01, then set the to_process = 1, which means that this invoice is not considered paid.
- Remove the row in payment_invoice that is now linking the payment and the invoice.
- A new method in the PaymentSessionBean is necessary to follow the facade pattern. It has to set the transaction as Required and just forward the call to PaymentBL.unapplyPayment.

Changes mostly to be done in these files:
view/payment/viewBody.jsp
PaymentSessionBean.java
PaymentBL.java
PaymentEntityBean.java
InvoiceEntityBean.java
PaymentInvoiceEntityBean.java (new file)
init.sql (to show the new database schema)
(com.sapienter.jbilling.client.payment) MaintainAction.java

This task is completed. These were the changes