c# - メインクエリ以外の変数にサブクエリを割り当てる必要があるのはなぜですか

Question

以下の GetTransfers() メソッドでは、GetAllocations() の結果をメイン クエリの外部の変数に割り当てる必要があります。そうしないと、クエリが失敗します。なぜ私はそれをしなければならないのですか？より良い方法はありますか？クエリが失敗すると、次のエラーが発生します。

{System.NotSupportedException: LINQ to Entities はメソッド 'System.Linq.IQueryable`1[XCBusinessLogic.Presentation.Allocation] GetAllocations()' メソッドを認識せず、このメソッドはストア式に変換できません。

このクエリは機能します:

public IQueryable<Transfer> GetTransfers()
    {
        IQueryable<Allocation> wxyz = GetAllocations();

        IQueryable<Transfer> query =
            from transfer in Context.XC_TRANSFERS
            //let wxyz = GetAllocations()
            join trader in Context.MGRS on transfer.TRADER_ID equals trader.MGR_NO
            join ssm in Context.SSM_CORES on transfer.SSM_ID equals ssm.SSM_ID
            join desk in Context.XC_DESKS on transfer.DESK_ID equals desk.DESK_ID

            select new Transfer
            {
                // snip
                _AllocationList = wxyz.Where(x => x.TRANSFER_ID == transfer.TRANSFER_ID)
            };
        return query;

    }

このクエリは失敗します:

public IQueryable<Transfer> GetTransfers()
    {
        //IQueryable<Allocation> wxyz = GetAllocations();

        IQueryable<Transfer> query =
            from transfer in Context.XC_TRANSFERS
            let wxyz = GetAllocations()
            join trader in Context.MGRS on transfer.TRADER_ID equals trader.MGR_NO
            join ssm in Context.SSM_CORES on transfer.SSM_ID equals ssm.SSM_ID
            join desk in Context.XC_DESKS on transfer.DESK_ID equals desk.DESK_ID

            select new Transfer
            {
                // snip
                _AllocationList = wxyz.Where(x => x.TRANSFER_ID == transfer.TRANSFER_ID)
            };
        return query;

    }

このクエリは失敗します:

public IQueryable<Transfer> GetTransfers()
    {
        //IQueryable<Allocation> wxyz = GetAllocations();

        IQueryable<Transfer> query =
            from transfer in Context.XC_TRANSFERS
            //let wxyz = GetAllocations()
            join trader in Context.MGRS on transfer.TRADER_ID equals trader.MGR_NO
            join ssm in Context.SSM_CORES on transfer.SSM_ID equals ssm.SSM_ID
            join desk in Context.XC_DESKS on transfer.DESK_ID equals desk.DESK_ID

            select new Transfer
            {
                // snip
                _AllocationList = GetAllocations().Where(x => x.TRANSFER_ID == transfer.TRANSFER_ID)
            };
        return query;

    }

GetAllocations メソッド:

public IQueryable<Allocation> GetAllocations()
    {
        IQueryable<Allocation> query =
            from alloc in Context.XC_ALLOCATIONS
            join acm in Context.ACMS on alloc.ACCT_NO equals acm.ACCT_NO
            join b in Context.BUM_DETAILS.Where(x => x.FIRM_NO == 1 && x.CATEGORY_ID == 1937) on acm.ACCT_NO equals b.ACCT_NO into bumDetails
            from bumDetail in bumDetails.DefaultIfEmpty()
            where acm.FIRM_NO == 1
            select new Allocation
            {
                AccountName = acm.ACCT_NAME
                // snip

            };
        return query;
    }