Question

我正在尝试在LINQ中执行多个表的连接。

我有以下课程：

public class Project {
    public int Id { get; set; }
    public string Name { get; set; }
}

public class Batch {
    public int Id { get; set; }
    public int ProjectId { get; set; }
    public Decimal Hours { get; set; }
    public Decimal DailyRate { get; set; }
}

public class TimeEntry
{
    public int Id { get; set; }
    public int BatchId { get; set; }
    public Decimal Expense { get; set; }
}

我正在使用以下LINQ表达式加入这些表，这些表达式就像一个魅力：

from project in _context.Projects
join batch in _context.Batches on project.Id equals batch.ProjectId into batchGroup from batch in batchGroup.DefaultIfEmpty()
join timeEntry in _context.TimeEntries on batch.Id equals timeEntry.BatchId into timeEntryGroup from timeEntriy in timeEntryGroup.DefaultIfEmpty()
select new {
    project,
    batch,
    timeEntriy
};

但是当我尝试对结果进行分组时，我得到了异常

from project in _context.Projects
join batch in _context.Batches on project.Id equals batch.ProjectId into batchGroup from batch in batchGroup.DefaultIfEmpty()
join timeEntry in _context.TimeEntries on batch.Id equals timeEntry.BatchId into timeEntryGroup from timeEntriy in timeEntryGroup.DefaultIfEmpty()
group  new {
    project,
    batch,
    timeEntriy
} by project.Id into g
select new {
    Id = g.Key,
    Hours = g.Sum(q => (q.batch == null ? 0 : q.batch.Hours))
};

// "Object reference not set to an instance of an object."

有人可以解释一下这里发生了什么或我错过了什么？

在LINQ中加入多个表并对结果进行分组

0 个答案: