如何遍历三个表并获取最大值

Question

In my EntityFramework I have three associated tables like so:

I'm trying to get the last time a file was used on a job. I have a value for File.Name and I need to get the maximum value for Job.ExecutedOn. I can obtain the File.FileId with a where clause, then in a separate expression get a list of FileHistory.JobId, then finally in a third expression get the maximum of Job.ExecutedOn. It feels very clumsy.

I was hoping for a single expression that accepts a value for File.Name and traverses across the tables to return the maximum value for Job.ExecutedOn. Is this possible? How?

Answer 1

This should do the trick

 DateTime GetLastExecutionTime(ObjectContext context, string fileName)
 {
    var query = from file in context.Files
                    join history in context.FileHistories 
                        on file.FileID equals history.FileID
                    join job in context.Jobs 
                        on history.JobID equals job.JobID
                    where file.FileName == fileName
                    select job.ExecutedOn;

    var result = query.Max();
 }

Answer 2

I believe this will work for you:

var query =
    from file in dc.Files
    where file.Name == fileName
    from history in file.FileHistories
    orderby history.Job.ExecutedOn descending
    select history.Job.ExecutedOn;
var lastExecutedDate = query.First();

I wrote it in a way so you could easily return the associated job instead. To do that just change the final projection to select history.Job .

Otherwise you could just remove the sort and take the Max() of the query (instead of First() ).