Linq to Entities: Getting the current selected row column to update the detail grid.

Question

After creating a linq to entity query and posting the query results to a Master Grid's Data Source, how can I update the detail Grid every time a user selects a different row in the Master grid?

For e.g. in the Form Load event I call the queries.

 int PageID = -1;
RackEntities re = new RackEntities();

private void PageList_Load( object sender, EventArgs e )
    {  
      var pageListQuery = from p in re.pages
                          where p.page_id != -1
                          select new { PageID = p.page_id };

      GridViewPage.DataSource = pageListQuery;

      var pageImageListQuery = from pi in re.page_image
                               where pi.page_id == PageID
                               select pi;

      GridViewPageImage.DataSource = pageImageListQuery;
    }

Then when the selection has changed in the Master Grid I want to update the Detail grid.

 

 private void GridViewPage_SelectionChanged( object sender, EventArgs e )
    {
      var pageImageListQuery = from pi in re.page_image
                               where pi.page_id ==  //PageID
                               select pi;

      GridViewPageImage.DataSource = pageImageListQuery;
    }