C# Linq2db:按嵌套属性字段筛选

我在项目中使用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;
}

Users表包含FullName列，我想知道是否可以按用户的全名筛选事务

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

---

是否可以使用linq2db实现它？

在工厂方法调用

事务后。构建（t，u，a）

，linq2db会丢失有关字段映射和查询的信息，这些信息只适用于具体化，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

如何使用linq2db使代码可重用-使用

ExpressionMethodAttribute

重写函数

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);
}

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
        }
    };