Determining Rows Inserted After A Period of Time

Question

I have a list of students in a SQL CE 4.0 database. Each one has an insert time associated with the student. Here is the context setup.

    public class Student
    {
        public int StudentId { get; set; }
        public string Name { get; set; }
        public DateTime InsertTime { get; set; }
    }

    public class StudentContext : DbContext
    {
        public StudentContext()
            : base("StudentContext")
        { }
        public DbSet<Student> Students { get; set; }
    }

Inserting a new student works great as shown below:

    static void AddStudent(string name)
    {
        using (var db = new StudentContext())
        {
            var s = new Student()
            {
                Name = name,
                InsertTime = DateTime.Now
            };
            db.Students.Add(s);
            db.SaveChanges();
        }
    }

The issue I am running into is that when I want to DELETE entries from the database after a given amount of time, using the linq to entities to query for entries that are older than a period of time fails. Below is my first attempt to look for entries older than 30 seconds.

        var student = (from x in db.Students
                       where x.Name == name
                       &&  x.InsertTime.AddSeconds(30) < DateTime.Now 
                       select x).ToList();

This fails with the following error:

LINQ to Entities does not recognize the method 'System.DateTime AddSeconds

Ok so apparently I need something linq will understand. Looking though StackOverflow I was able to find a method called EntityFunctions and modified my code as follows:

System.Data.Objects.EntityFunctions.AddSeconds(x.InsertTime, 30) < DateTime.Now 

This also fails with the following error:

The function is not recognized by SQL Server Compact. [ Name of function = ADDSECONDS...

Great, after more digging I found that the EntityFunctions only work with the SQL Provider, and not the SQL CE 4.0 provider. Perfect!

The only thing I could come up with was getting the list of entries and then filtering that collection once again.

  var student = (from x in db.Students
                               where x.Name == name  
                               select x).ToList();

  var filteredStudent = student.Where(s => s.InsertTime.AddSeconds(30) < DateTime.Now).ToList();

This does work and the correct data is now available for deletion. However, the query could potentially return a large set of data, only to be filtered down to a couple of entries. The ADDDATE() is supported by Sql Compact CE 4.0. Is there any way I could write my own extension to the Linq to entities that would allow me to utilize the function? Can you think of another way to solve this problem?

Answer 1

Could you try something like this? This should take the time manipulation out of the linq.

DateTime time = DateTime.Now.AddSeconds(-30)

var student = (from x in db.Students
              where x.Name == name &&  x.InsertTime < time
              select x).ToList();