entity-framework - GroupBy 表达式无法翻译

Question

//Model
public class Application
{
    [Key]
    public int ApplicationId { get; set; }
    public DateTime CreatedAt { get; set; }
    public DateTime ConfirmedDate { get; set; }
    public DateTime IssuedDate { get; set; }
    public int? AddedByUserId { get; set; }
    public virtual User AddedByUser { get; set; }
    public int? UpdatedByUserId { get; set; }
    public virtual User UpdatedByuser { get; set; }
    public string FirstName { get; set; }
    public string MiddleName { get; set; }
    public string LastName { get; set; }
    public string TRN { get; set; }
    public string EmailAddress { get; set; }
    public string Address { get; set; }
    public int ParishId { get; set; }
    public Parish Parish { get; set; }
    public int? BranchIssuedId { get; set; }
    public BranchLocation BranchIssued { get; set; }
    public int? BranchReceivedId { get; set; }
    public BranchLocation BranchReceived {get; set; }
}

public async Task<List<Application>> GetApplicationsByNameAsync(string name)
{
    if (string.IsNullOrEmpty(name))
        return null;
    return await _context.Application
        .AsNoTracking()
        .Include(app => app.BranchIssued)
        .Include(app => app.BranchReceived)
        .Include(app => app.Parish)
        .Where(app => app.LastName.ToLower().Contains(name.ToLower()) || app.FirstName.ToLower()
        .Contains(name.ToLower()))
        .GroupBy(app => new { app.TRN, app })
        .Select(x => x.Key.app)
        .ToListAsync()
        .ConfigureAwait(false);
}

上述 GroupBy 表达式无法在 VS Studio 中编译。我的目标是按包含用户给定字符串的名称运行查询过滤结果，然后它应该通过类似的 TRN 数字对结果进行分组，返回这些应用程序的列表以返回到 View 。我想我真的很接近，但似乎无法弄清楚查询的最后一点。任何指导表示赞赏。

出现错误

InvalidOperationException: The LINQ expression 'DbSet<Application>
.Where(a => a.LastName.ToLower().Contains(__ToLower_0) || a.FirstName.ToLower().Contains(__ToLower_0))
.GroupBy(
source: a => new {
TRN = a.TRN,
app = a
},
keySelector: a => a)' could not be translated. Either rewrite the query in a form that can be translated, or switch to client evaluation explicitly by inserting a call to either AsEnumerable(), AsAsyncEnumerable(), ToList(), or ToListAsync()

更新
似乎这绝对是由于自最近更新以来 .net core 3.x 和 EF core 一起玩的方式发生了变化。我不得不使用 AsEnumerable() 而不是 ToListAsync() 将其更改为客户端评估。 Steve py 给出的查询的其余部分使用此方法。即使在阅读文档后，我也不知道 groupby 是如何在 LINQ 中真正工作的，所以这对我有很大帮助。然而，将查询带到客户端 eval 可能会出现性能问题。

Answer 1

基于此:

I want to group by TRN which is a repeating set of numbers eg.12345, in the Application table there may be many records with that same sequence and I only want the very latest row within each set of TRN sequences.

我相信这应该可以满足您的需求:

return await _context.Application
    .AsNoTracking()
    .Include(app => app.BranchIssued)
    .Include(app => app.BranchReceived)
    .Include(app => app.Parish)
    .Where(app => app.LastName.ToLower().Contains(name.ToLower()) || app.FirstName.ToLower()
    .Contains(name.ToLower()))
    .GroupBy(app => app.TRN)
    .Select(x => x.OrderByDescending(y => y.CreatedAt).First())
    .ToListAsync()
    .ConfigureAwait(false);

GroupBy 表达式应该代表你想要的分组依据。在你的情况下，TRN。从那里开始，当我们进行选择时， x 代表每个“组”，其中包含属于每个 TRN 的可枚举应用程序集。因此，我们按 CreatedAt 日期的降序对它们进行排序，以使用 First 选择最新的。

试一试。如果这不是您想要的，请考虑为您的问题添加一个示例集，以及所需的输出与此处产生的输出/错误。