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？
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,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,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
}
};