Question

我在项目中使用linq2db ORM，并且有以下查询：

pushstate

和Build方法如下：

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);

“用户”表包含“全名”列，我想知道是否可以按用户的全名来过滤交易吗？

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

是否可以用linq2db实现它？

Answer 1

在工厂方法调用Transaction.Build(t, u, a)之后，linq2db丢失了有关字段映射的信息，查询变得仅对实现有利，而对过滤却无济于事。对于任何当前可用的LINQ提供程序都是如此。

使用linq2db可以使代码可重用的方法-使用Transaction.Build重写函数ExpressionMethodAttribute：

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);
}

进行这些操作后，可以在Transaction.Build调用之后过滤查询。

在后台，linq2db将找到ExpressionMethodAttribute声明，并用Transaction.Build函数中定义的Expression替换BuildImpl调用。在分析LINQ查询之前，将其示意图化为以下变体：

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
        }
    };

Linq2db：按嵌套属性字段过滤

1 个答案: