Datatable as row another Datatable

[Pipin]

Hi, All!
I have to realize a document which have a header and Lines(for example Purchase Order). The data from header is based on table "PurchaseTable" from database, the Lines are based on another table "PurchaseLine". In database i have related this tables like OneToMany. Next in Netbeans i have created Entity Clasess using persistence(JPA) and JSF pages with primefaces datatable component. But i couldn't understand how i can create in one JSF page two related p:datable where lines in second datatable(PurchaseLine) depends from row which is selected in first datatable(Purchase Order)!
What should i do?Can anyone help me?
I'am using Netbeans 7.0 and primefaces 2.2.1