實體框架核心：如何通過僅選擇最新創建的記錄在“ IQueryable”上執行聯接？

Question

我正在重構生產中重復的代碼：

private IQueryable<CashoutModel> GetCashoutModels() =>
    from cashout in _context.Cashouts
    join audit in _context.Audits
        on cashout.Id.ToString() equals audit.EntityId
        into cashoutModel
    from audit in cashoutModel.DefaultIfEmpty()
    orderby cashout.CreatedOn descending
    select new CashoutModel
    {
        Id = cashout.Id,
        Amount = cashout.Amount,
        Comment = cashout.Comment,
        CreatedOn = cashout.CreatedOn,
        RecipientAccountId = cashout.RecipientAccountId,
        RecipientAccountName = cashout.RecipientAccountName,
        State = cashout.State,
        Reason = cashout.Reason,
        CreatedBy = audit == null 
            ? null 
            : audit.Name
    };

_context.Audits實際上在_context.Cashout記錄了某些記錄發生的（使用觸發）更改。

我要尋找一個干凈的方式來解決的當前行為join只選擇（對於給定EntityId ）最新的審計記錄。

Answer 1

您當前擁有的代碼實際上是在做一個LEFT OUTER JOIN ，它獲取所有Cashouts記錄和相應的審計記錄。

假設您在“ Audits表中有一個時間戳列（例如CreatedOn ）捕獲創建該審計記錄的時間，則可以在此處使用GROUP JOIN ，如下所示：

private IQueryable<CashoutModel> GetCashoutModels() =>
_context.Cashouts.GroupJoin(_context.Audits,
                            c => c.Id.ToString(),
                            a => a.EntityId,
                            (cashout, audit) => new CashoutModel
                                {
                                    Id = cashout.Id,
                                    Amount = cashout.Amount,
                                    Comment = cashout.Comment,
                                    CreatedOn = cashout.CreatedOn,
                                    RecipientAccountId = cashout.RecipientAccountId,
                                    RecipientAccountName = cashout.RecipientAccountName,
                                    State = cashout.State,
                                    Reason = cashout.Reason,
                                    CreatedBy = audit.OrderByDescending(x => x.CreatedOn)
                                                    .Select(y => y.Name)
                                                    .FirstOrDefault()
                                }).AsQueryable();