C#, LINQ: IQueryable with Tuple Extension Method

OrderHeaderExtensions.cs

using System;
using System.Linq;
using System.Linq.Expressions;

namespace Fox.Xavier.Models.Extensions
{
    using Fox.Entities;
    using Fox.Xavier.Models;

    /// <summary>
    /// Extensions of <see cref="OrderHeader"/>
    /// </summary>
    public static class OrderHeaderExtensions
    {
        /// <summary>
        /// Converts the <see cref="OrderHeader" /> into a <see cref="Requisition"/>.
        /// </summary>
        /// <remarks>
        /// This did not work with EF when:
        ///     i) An attempt was made to cast to Tuple&ltOrderHeader,OrderStatus&gt;
        ///     ii) An attempt was made to join with Tuple&ltOrderHeader,OrderStatus&gt;
        /// </remarks>
        public static IQueryable<Requisition> ToRequisition(this IQueryable<Tuple<OrderHeader,OrderStatus>> queryable)
        {
            Expression<Func<Tuple<OrderHeader, OrderStatus>, Requisition>> storeExpression =
                input => new Requisition
                {
                    OrderId = input.Item1.OrderId,
                    OrderNumber = input.Item1.OrderNumber,
                    PONumber = input.Item1.PONumber,
                    OrderStatusID = input.Item1.OrderStatusID,
                    DepartmentID = input.Item1.DepartmentID,
                    CustomerName = input.Item1.CustomerName,
                    CreatedDt = input.Item1.CreatedDt,
                    PriorityFl = input.Item1.PriorityFl,
                    OrderStatusDescription = input.Item2.StatusDescription,
                    CreatedBy = input.Item1.CreatedBy,
                    OrderTotal = input.Item1.OrderDetails.Sum(i => i.TotalAmount)
                };

            return queryable.Select(storeExpression);
        }
    }
}