How to populate dropdown list value based on another in edit

Question

Hi,
I've 2 dropdowns 1 is site and another is warehouse. Warehouse dropdown value will be changed based on site dropdown.

I done the insert for that dropdowns but when I try to edit the dropdowns already existing value need to be shown and dropdown value should change based on the site edit control..

How could i do the edit for this 2 controls?
I couldn't find a perfect example for this, can anyone help me in this?

What I have tried:

This is my insert template:

<asp:DropDownList ID= "TxtsiteInsert" runat="server" AutoPostBack="True" OnSelectedIndexChanged="sitedropdown_SelectedIndexChanged" TabIndex="6">

<asp:ListItem Text="Select Site"></asp:ListItem>

</asp:DropDownList>

</InsertItemTemplate>

<InsertItemTemplate>

<asp:DropDownList ID= "TxtWarehouseInsert" runat="server" AutoPostBack="True" TabIndex="7" >

<asp:ListItem Text="Select warehouse"></asp:ListItem>

</asp:DropDownList>

</InsertItemTemplate>

This is my coding behind that:

Protected Sub sitedropdown_SelectedIndexChanged(ByVal sender As Object, ByVal e As EventArgs)

Dim IdTxtsiteInsert As DropDownList = CType(DetailsView3.Rows(0).FindControl("TxtsiteInsert"), DropDownList)

Fillwarehouse(IdTxtsiteInsert.Text)

End Sub

Public Sub Fillwarehouse(ByVal siteid As String)

'Dim IdTxtWarehouseInsert As DropDownList = CType(DetailsView3.Rows(0).FindControl("TxtWarehouseInsert"), DropDownList)

Dim ddr1 As DropDownList = CType(DetailsView3.Rows(0).FindControl("TxtWarehouseInsert"), DropDownList)

Dim strConn As String = WebConfigurationManager.ConnectionStrings("BMGINC_DYNAX09_ProdConnectionString").ConnectionString

Dim con As New SqlConnection(strConn)

Dim cmd As New SqlCommand()

cmd.Connection = con

cmd.CommandType = CommandType.Text

cmd.CommandText = "Select InventLocationId, Name from InventLocation where dataareaid='BRM' and InventSiteId =@InventSiteId"

cmd.Parameters.AddWithValue("@InventSiteId", siteid)

Dim objDs As New DataSet()

Dim dAdapter As New SqlDataAdapter()

dAdapter.SelectCommand = cmd

con.Open()

dAdapter.Fill(objDs)

con.Close()

If objDs.Tables(0).Rows.Count > 0 Then

ddr1.DataSource = objDs.Tables(0)

ddr1.DataTextField = "InventLocationId"

ddr1.DataValueField = "InventLocationId"

ddr1.DataBind()

ddr1.Items.Insert(0, "-Select warehouse-")

Else

ddr1.DataTextField = " "

ddr1.DataValueField = " "

ddr1.DataBind()

ddr1.Items.Insert(0, " ")

LblError.Text = "No Warehouse found"

End If

End Sub