title: Filter form's main datasource through the linked datasource
body: <p>I have a custom form which has two datasouces. Lets say to make it easer that  my form has the Salestable and SalesLines datasources. </p>
  
  <p>for example I could say that I have a filter which is bounded with the ItemGroup edt .</p>
  
  <p>I want to filter the SalesTable Datasource through this filter in order to 'show' in a grid which is connected with the SalesOrders datasouce : </p>
  
  <p><em>''all SalesOrders which 'have' saleslines with  saleslines.ItemGroup == somethingfromFilter</em>''.</p>
  
  <p>Whatever I tried is faild. Can someone help me?</p>
  
  <p>FYI: the datasources properties I asume that  are proper linked: 
  SalesLine.JoinSource = SalesTable
  All of my tries was in modified method of the filter. </p>