Master/DetailFilteringWithTwoDropDownListsIntroductionIntheHYPERLINK"http://www.asp.net/learn/data-access/tutorial-07-cs.aspx"previoustutorialweexaminedhowtodisplayasimplemaster/detailsreportusingasingleDropDownListpopulatedwiththecategoriesandaGridViewshowingthoseproductsthatbelongtotheselectedcategory.Thisreportpatternworkswellwhendisplayingrecordsthathaveaone-to-manyrelationshipandcaneasilybeextendedtoworkforscenariosthatincludemultipleone-to-manyrelationships.Forexample,anorderentrysystemwouldhavetablesthatcorrespondtocustomers,orders,andorderlineitems.Agivencustomermayhavemultipleorderswitheachorderconsistingofmultipleitems.SuchdatacanbepresentedtotheuserwithtwoDropDownListsandaGridView.ThefirstDropDownListwouldhavealistitemforeachcustomerinthedatabasewiththesecondone'scontentsbeingtheordersplacedbytheselectedcustomer.AGridViewwouldlistthelineitemsfromtheselectedorder.WhiletheNorthwinddatabaseincludethecanonicalcustomer/order/orderdetailsinformationinitsCustomers,Orders,andOrderDetailstables,thesetablesaren'tcapturedinourarchitecture.Nonetheless,wecanstillillustrateusingtwodependentDropDownLists.ThefirstDropDownListwilllistthecategoriesandthesecondtheproductsbelongingtotheselectedcategory.ADetailsViewwillthenlistthedetailsoftheselectedproduct.Step1:CreatingandPopulatingtheCategoriesDropDownListOurfirstgoalistoaddtheDropDownListthatliststhecategories.Thesestepswereexaminedindetailintheprecedingtutorial,butaresummarizedhereforcompleteness.OpentheMasterDetailsDetails.aspxpageintheFilteringfolder,addaDropDownListtothepage,setitsIDpropertytoCategories,andthenclicktheConfigureDataSourcelinkinitssmarttag.FromtheDataSourceConfigurationWizardchoosetoaddanewdatasource.Figure1:AddaNewDataSourcefortheDropDownList(HYPERLINK"http://static.asp.net/asp.net/images/dataaccess/08fig01cs.png"Clicktoviewfull-sizeimage)Thenewdatasourceshould,naturally,beanObjectDataSource.NamethisnewObjectDataSourceCategoriesDataSourceandhaveitinvoketheCategoriesBLLobject'sGetCa