Bind data to a DropDownList in Edit & Insert in ListView

Question

Hi, 

    In my ASP.NET 4.5 WebForms app, In my ListView I am trying to bind DropDownList, but am not able to do it. Actually, the Model that is bind to ListView, has a Property of UnitdirectionId & on Edit & Insert Templates' I want to show the dropdown of all UnitDirection table.

Here is my EditTempalte :

            <EditItemTemplate>

               <tr>

                   <td>

                       <asp:TextBox ID="unitDirTxt" runat="server" Text='<%# Bind("UnitDirectionId") %>'></asp:TextBox>

                        <!-- <asp:DynamicControl runat="server" DataField="UnitDirectionId" ID="UnitDirectionId" Mode="Edit" />  -->

                       <asp:DropDownList ID="unitDirEditDrop" runat="server" ItemType="VincitoreCRMApplication.Models.UnitDirection"

                           DataTextField="DirectionTitle" DataValueField="UnitDirectionId" 

                           SelectMethod="GetUnitDirections"></asp:DropDownList>  

                   </td> 

              </tr>

            </EditItemTemplate>

In Code behind, 

        public IQueryable<UnitDirection> GetUnitDirections()

        {

            return _db.UnitDirections;

        }

Now, the Model of LsitView has the proerpty as follows :

        [Display(Name= "Unit Direction")]

        public int UnitDirectionId { get; set; }

        [ForeignKey("UnitDirectionId")]

        public virtual UnitDirection UnitDirection { get; set; }

Now, the UnitDirection Model :

    public class UnitDirection

    {

        [ScaffoldColumn(false)]

        public int UnitDirectionId { get; set; }

        [Required]

        [Display]

        public string DirectionTitle { get; set; }

    }

In db the UnitDirection table has 3 items. 

With the  above drop down, I believe I should atleast get list of all UnitDirection in drop down. But on that I am getting error. Then comes to set the selected item in Edit Template.

Can you please help me identify why I am getting this error ?? 
Any help is highly appreciated.

Thanks