Master-Detail form query and update

Hi

i am working on a master-detail form, where i have the requirement as below:

1. Header(master) details will be fetched from a standard header table.

2. Details data will we queried first time from the standard lines table, then some information will be updated in the form fields and data will be inserted into a custom table along with the header id of the master standard table.

I have created two data-blocks for each of the table. I have a requirement that when i select a value in the master block, the detail block should auto populate, but since i have kept the data source for detail block as