Update Grid row

I have a DataGrid where four fields are shown. ( First name, Last name, Email, Membership Name).

I have a form shown on right side of grid. When I select a row from grid the relevant data is set to form fields. There is a submit button in form, When i click on button the row or form updates all value. In form i have a combo element that is connected with (membership) table and all other fields First name, Last name and Email are connected with other table(user). Member table id is used as foreign key in user table. Problem is that when i update form the foreign key (membership_id) in user table is not updated. While its updated in membership table. I want to update just user table when combo value (membership) is changed. I do not want to update membership table. Please help me out.

You may try to use the setDataColumns() dataprocessor method. It allows to define which column may trigger update.
which column may trigger update:
If the problem still occurs for you please, provide a snippet with your using code.