在 EfCore 的相关模型中通过 Sum 聚合有效地查询/排序

Question

I'm still fairly new to C#, .NET and EfCore so please bear with me and apologies if I'm missing something in the question.

Let's say I have the following relations

public class User
{
    public Account Account {get; set;}
    public string Foo {get; set;}

}

public class Account
{
    public List<Transaction> Transactions {get; set;}
}

public class Transaction
{
    public decimal Amount {get; set;}
    public TransactionType TransactionType {get; set;}

}

public enum TransactionType 
{
    Credit = 1,
    Debit = 2,
    Refund = 3,
}

I'd like to work with balances, which need to be calculated every time the User model is retrieved. For this example, let's say I need to order a list of users, where Foo is "Bar", by their account balance

var query = db.Users
    .Include(u => u.Account)
    .ThenInclude(a => a.Transactions)
    .Where(u => u.Foo == "Bar");

var orderedQuery = query
         .OrderByDescending(u => 
              (u.Account.Transactions
                .Where(t => t.TransactionType == TransactionType.Credit || t.TransactionType == TransactionType.Refund)
                .Sum(t => t.Amount)) 
              - u.Account.Transactions
                 .Where(t => t.TransactionType == TransactionType.Debit)
                 .Sum(t => t.Amount)
              )
);


// Build the List
return orderedQuery.Skip(...).Take(...).Select(x => new SomeDTO{/* Build some DTO's with User Info, Account Info and Balance */}).ToList();

The above works, but is inefficient.

I'm working on a fairly large codebase and existing database in a asp.net core & sql-server project, so I need to work with what I've got. Can anyone suggest a better way of doing this kind of work?

Answer 1

Do you really need all these Include s? They are not necessary for the ordering aggregation later.

var query = db.Users
    .Include(u => u.Account)
    .ThenInclude(a => a.Transactions)
    .Where(u => u.Foo == "Bar");

---

You can optimize the ordering part of the query, by combining into a single grouping

var orderedQuery = query.OrderByDescending(u =>
    u.Account.Transactions
    .GroupBy(t => 1)
    .Select(g =>
          g.Where(t => t.TransactionType == TransactionType.Credit || t.TransactionType == TransactionType.Refund)
           .Sum(t => t.Amount)
        - g.Where(t => t.TransactionType == TransactionType.Debit)
           .Sum(t => t.Amount)
        )
    );

Skip Take is pretty inefficient, as it requires searching through the whole list again. It may be prudent to cache it all in the client and page it afterwards.