Join and group by in LINQ - entity-framework

Question

Here is the entity and table structure-
class Person { int PersonId, string PersonName}
class Report { int ReportId, datetime ReportTime, int PersonId }
Table: Persons
----------------------------
| PersonId | PersonName |
----------------------------
| 1 | Abc |
----------------------------
| 2 | Xyz |
----------------------------
Table: Reports
----------------------------------------------
| ReportId | ReportTime | PersonId |
----------------------------------------------
| 10 | 2017-02-27 11:12 | 1 |
---------------------------- -----------------
| 14 | 2017-02-27 15:23 | 1 |
---------------------------- -----------------
I want to select data as follows (PersonName from Person and last record of his Id in reports table)-
-------------------------------------
| PersonName | ReportTime |
-------------------------------------
| Abc | 2017-02-27 15:23 |
-------------------------------------
How can i do it in Lambda or LINQ?

Answer 1

Use Queryable.GroupJoin:
from p in db.Persons
join r in db.Reports on p.PersonId equals r.PersonId into g
where g.Any() // if some persons do not have reports
select new {
p.PersonName,
ReportTime = g.Max(r => r.ReportTime)
}
Lambda (note that it will return Nullable ReportTime with nulls for persons which don't have any reports)
db.Persons.GroupJoin(
db.Reports,
p => p.PersonId,
r => r.PersonId,
(p,g) => new { p.PersonName, ReportTime = g.Max(r => (DateTime?)r.ReportTime) })

Answer 2

Try this:
List<Person> people = new List<Person>
{
new Person {PersonId = 1, PersonName = "AB" },
new Person {PersonId = 2, PersonName = "CD" },
new Person {PersonId = 3, PersonName = "EF" },
};
List<Report> reports = new List<Report>()
{
new Report {ReportId = 1, ReportTime = DateTime.Now, PersonId = 1 },
new Report {ReportId = 2, ReportTime = DateTime.Now.AddHours(-1), PersonId = 1 },
new Report {ReportId = 3, ReportTime = DateTime.Now.AddHours(-2), PersonId = 1 },
new Report {ReportId = 4, ReportTime = DateTime.Now.AddMinutes(-3), PersonId = 2 },
new Report {ReportId = 5, ReportTime = DateTime.Now, PersonId = 2 }
};
var res = (from rep in reports
group rep by rep.PersonId into repGrp
join ppl in people on repGrp.FirstOrDefault().PersonId equals ppl.PersonId
select new
{
PersonName = ppl.PersonName,
ReportDate = repGrp.Max(r => r.ReportTime),
}).ToList();
}