LINQ Join Group By Select New - linq

Question

I have the following entities:
User is:
public String Id { get; set; }
public String FirstName { get; set; }
public String LastName { get; set; }
Assessment is:
public int Id { get; set; }
public int SymptomId { get; set; }
public string UserId { get; set; }
public DateTime CreatedDate { get; set; }
A User can have 0, 1 or more Assessments.
I have written the following LINQ:
// Get Paged Users Recent Assessment List:
var _usersWithRecentAssessment =
from U in _context.Users
join A in _context.Assessments on U.Id equals A.UserId
group A by A.UserId into uaGroup
select uaGroup.OrderByDescending(a => a.CreatedDate).FirstOrDefault();
_usersWithRecentAssessment = _usersWithRecentAssessment.OrderByDescending(ua => ua.CreatedDate);
which returns the most recent symptom assessment for all Users that have completed an Assessment (and orders the assessment list in descending order of Assessment CreatedDate) as follows:
[
{
"id": 1052,
"symptomId": 44,
"userId": "b978d113-7da7-4b7f-a121-9dd71e158dd4",
"createdDate": "2019-11-16T12:50:05.2175621"
},
{
"id": 1051,
"symptomId": 44,
"userId": "5230f4b7-bf2a-46b0-88a0-6f13fa5caa91",
"createdDate": "2019-11-03T14:46:21.6598763"
}
]
I would like to return the following AssessmentDTO
where AssessmentDTO is:
public int Id { get; set; }
public int SymptomId { get; set; }
public string UserId { get; set; }
public string UserFirstName { get; set; }
public string UserLastName { get; set; }
public DateTime CreatedDate { get; set; }
which contains the additional attributes UserId, UserFirstName, and UserLastName from the User entity.
I have tried unsuccessfully to add 'select new AssessmentDTO() { }' at the end of the LINQ.
Can someone please help me?

Answer 1

A User has zero or more Assessments, every Assessment belongs to exactly one User. This is a standard one-to-many relation with a foreign key.
For your problem, you can use one of the overload of Queryable.GroupBy. First get all Users with their assessments and as a result keep only the most recent assessment:
var UsersWithMostRecentAssessment = dbContext.Users
.GroupBy(dbContext.Assessments, // GroupJoin Users and Assessments
user => user.Id, // From every User take the Id
assessment => assessment.UserId, // From every Assessment take the UserId
// ResultSelector: take each User with its zero or more Assessments to make one new:
(user, assessmentsOfThisUser) => new
{
User = user,
MostRecentAssessment = assessmentsOfThisUser
.OrderByDescending(assessment => assessment.CreatedDate)
.FirstOrDefault(),
// might be null if this User has not assessments at all
})
// Now get the User with its MostRecentAssessment (or null) to make one new:
.Select(userAssessMent => new
{
// Values from the most recent assessment:
Id = userAssessment.MostRecentAssessment.Id,
SymptomId = userAssessment.MostRecentAssessment.SymptomId,
...
// Values from the User:
UserId = userAssessment.User.Id,
FirstName = userAssessment.User.FirstName,
LastName = userAssessment.User.LastName,
...
})
Note: this will go wrong if there are users without assessments, because they won't have a most recent assessment. You can omit these users before the final Select:
.Where(userAssessment => userAssessment.MostRecentAssessment != null)
Or if you want these Users in your endresult:
// Values from the most recent assessment or default if null
Id = userAssessment.MostRecentAssessment.Id ?? 0,
SymptomId = userAssessment.MostRecentAssessment.SymptomId ?? 0,