How to add a child table graph to EntityDataSource

I have an EntityDataSource working to get the row data from tblOrderFile like this:

<asp:EntityDataSource ID="entityDataSourcePreorder" runat="server" 
        ConnectionString="name=iDBEntities" 
        DefaultContainerName="iDBEntities" EnableFlattening="False" 
        EntitySetName="tblOrderFiles" 
        Select="it.[pkOrderFileID], it.[fkOrderFileStatusID], it.[Filename], it.[CreateDate], it.[UserId]" 
        AutoGenerateWhereClause="True" EntityTypeFilter="" Where="">

      

        
        
        
      

    

Now I would like to change it to return the number of rows in the tblOrderFileItem child table (named tblOrderFileItems).

I found a way to make Count work by adding an Include directive like this:

    <asp:EntityDataSource ID="entityDataSourcePreorder" runat="server" 
        ConnectionString="name=iDBEntities" 
        DefaultContainerName="iDBEntities" EnableFlattening="False" 
        EntitySetName="tblOrderFiles" Include="tblOrderFileItems"
        AutoGenerateWhereClause="True" EntityTypeFilter="" Where="" >
    </asp:EntityDataSource>

      

        
        
        
      

    

but I believe this returns all columns of all rows for each order item. I just want to graph and don't want to deliver the rest of the data to the webpage.

I also tried just adding it.tblOrderFileItems.Count to the Select statement, but get the error

'Count' is not a member of 'Transient.collection [MyDBModel.tblOrderFileItem (Nullable = True, DefaultValue =)]'. To retrieve a property of a collection item, use a subquery to iterate over the collection.