Linq2db：按嵌套属性字段过滤

Question

我在我的项目中使用了 linq2db ORM，我有以下查询：

var query =
        from t in _db.Transactions
        from a in _db.Accounts.LeftJoin(a => a.Id == t.AccountId)
        from u in _db.Users.LeftJoin(u => u.Id == a.UserId)
        select Transaction.Build(t, u, a);

构建方法如下所示：

public static Transaction Build(Transaction transaction, User user, Account account)
{
  account.User = user;
  transaction.Account = account;
  return transaction;
}

用户 table 包含全名列，我想知道是否可以按用户全名过滤交易？

if (!string.IsNullOrWhiteSpace(userNameFilter))
{
  query = query.Where(t => t.Account.User.FullName.Contains(userNameFilter));
}

可以用linq2db实现吗？

Answer 1

在工厂方法调用后 Transaction.Build(t, u, a)，linq2db 丢失了有关字段映射的信息，并且查询变得仅适用于物化而不适用于过滤。对于任何当前可用的 LINQ 提供程序都是如此。

您可以使用 linq2db 做什么来使代码可重用 - 使用 ExpressionMethodAttribute:

重写您的函数 Transaction.Build

static Func<Transaction, User, Account, Transaction> _buildFunc;

static Expression<Func<Transaction, User, Account, Transaction>> BuildImpl()
{
   return (Transaction transaction, User user, Account account) => 
      new Transaction
      {
         Id = transaction.Id,
         ... // copy all needed fields

         Account = new Account 
         {
            Id = account.Id,
            ... // copy all needed fields

            User = user
         }
      }
   );
}

[ExpressionMethod(nameof(BuildImpl))]
public static Transaction Build(Transaction transaction, User user, Account account)
{
   // we left function usable even for non-query scenarios.
   _buildFunc ??= BuildImpl().Compile();
   return _buildFunc(transaction, user, account);
}

经过这些操作后，您的查询可以在 Transaction.Build 调用后进行过滤。

在引擎盖下，linq2db 将找到 ExpressionMethodAttribute 声明并用 BuildImpl 函数中定义的表达式替换 Transaction.Build 调用。示意性地在分析 LINQ 查询之前，将其转换为以下变体：

var query =
    from t in _db.Transactions
    from a in _db.Accounts.LeftJoin(a => a.Id == t.AccountId)
    from u in _db.Users.LeftJoin(u => u.Id == a.UserId)
    select new Transaction
    {
        Id = t.Id,
        ... // copy all needed fields

        Account = new Account 
        {
            Id = a.Id,
            ... // copy all needed fields

            User = u
        }
    };

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