使用实体框架连接多个表答案

【问题标题】：Joining multiple tables using Entity Framework使用实体框架连接多个表
【发布时间】：2012-10-30 15:05:39
【问题描述】：

我正在尝试使用 EF 加入 3 个表，但它会引发错误提示

consider swaping conditions on either side of equals

有人可以帮忙

 var billdata = from billtotal in context.billTotals
                                   join billcard in context.billClubcards
                                       on billtotal.OrderID equals billcard.OrderID

                                   join billtender in context.billTenders
                                       on billtender.OrderID equals billtotal.OrderID


                                   select billtotal;

【问题讨论】：

标签： c# linq entity-framework

【解决方案1】：

编译器错误非常正确：-

The name 'billtender' is not in scope on the left side of 'equals'. Consider swapping the expressions on either side of 'equals'.

您要加入的表需要在左侧，您要加入的表需要在右侧。因此：-

var billData =
    from billtotal in context.billTotals
    join billcard in context.billClubcards
        on billtotal.OrderId equals billcard.OrderId
    join billtender in context.billTenders
        on billtotal.OrderId equals billtender.OrderId
    select billtotal;

如果你想知道为什么，那是因为查询语法只是底层扩展方法的语法糖：-

context.billTotals
  .Join(
    context.billClubcards,
    billtotal => billtotal.OrderId,
    billclubcard => billclubcard.OrderId,
    (billtotal, billclubcard) => billtotal)
  .Join(
    context.billTenders,
    billtotal => billtotal.OrderId,
    billtender => billtender.OrderId,
    (billtotal, billtender) => billtotal);

您的原始实现将扩展为：-

context.billTotals
  .Join(
    context.billClubcards,
    billtotal => billtotal.OrderId,
    billclubcard => billclubcard.OrderId,
    (billtotal, billclubcard) => billtotal)
  .Join(
    context.billTenders,
    billtotal => billtender.OrderId, // billtender isn't in scope!
    billtender => billtotal.OrderId, // billtotal isn't in scope!
    (billtotal, billtender) => billtotal);

【讨论】：

【解决方案2】：

你要加入的表的属性需要在equals的右边

切换

来自：on billtender.OrderID equals billtotal.OrderID

收件人：on billtotal.OrderID equals billtender.OrderID

【讨论】：