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How to write linq query for this sql statement

How would you write a linq query with the following SQL statement. I've tried several methods referenced on stackoverflow but they either don't work with the EF version I'm using (EF core 3.5.1) or the DBMS (SQL Server).
select a.ProductID, a.DateTimeStamp, a.LastPrice
from Products a
where a.DateTimeStamp = (select max(DateTimeStamp) from Products where a.ProductID = ProductID)
For reference, a couple that I've tried (both get run-time errors).
var results = _context.Products
.GroupBy(s => s.ProductID)
.Select(s => s.OrderByDescending(x => x.DateTimeStamp).FirstOrDefault());
var results = _context.Products
.GroupBy(x => new { x.ProductID, x.DateTimeStamp })
.SelectMany(y => y.OrderByDescending(z => z.DateTimeStamp).Take(1))
Thanks!

I understand you would like to have a list of the latest prices of each products?
First of all I prefer to use group by option even over 1st query
select a.ProductID, a.DateTimeStamp, a.LastPrice
from Products a
where a.DateTimeStamp IN (select max(DateTimeStamp) from Products group by ProductID)
Later Linq:
var maxDateTimeStamps = _context.Products
.GroupBy(s => s.ProductID)
.Select(s => s.Max(x => x.DateTimeStamp)).ToArray();
var results = _context.Products.Where(s=>maxDateTimeStamps.Contains(s.DateTimeStamp));
-- all assuming that max datetime stamps are unique

I've managed to do it with the following which replicates the correlated sub query in the original post (other than using TOP and order by instead of the Max aggregate), though I feel like there must be a more elegant way to do this.
var results = from x
in _context.Products
where x.DateTimeStamp == (from y
in _context.Products
where y.ProductID == x.ProductID
orderby y.DateTimeStamp descending
select y.DateTimeStamp
).FirstOrDefault()
select x;

I prefer to break up these queries into IQueryable parts, do you can debug each "step".
Something like this:
IQueryable<ProductOrmEntity> pocoPerParentMaxUpdateDates =
entityDbContext.Products
//.Where(itm => itm.x == 1)/*if you need where */
.GroupBy(i => i.ProductID)
.Select(g => new ProductOrmEntity
{
ProductID = g.Key,
DateTimeStamp = g.Max(row => row.DateTimeStamp)
});
//// next line for debugging..do not leave in for production code
var temppocoPerParentMaxUpdateDates = pocoPerParentMaxUpdateDates.ToListAsync(CancellationToken.None);
IQueryable<ProductOrmEntity> filteredChildren =
from itm
in entityDbContext.Products
join pocoMaxUpdateDatePerParent in pocoPerParentMaxUpdateDates
on new { a = itm.DateTimeStamp, b = itm.ProductID }
equals
new { a = pocoMaxUpdateDatePerParent.DateTimeStamp, b = pocoMaxUpdateDatePerParent.ProductID }
// where
;
IEnumerable<ProductOrmEntity> hereIsWhatIWantItems = filteredChildren.ToListAsync(CancellationToken.None);
That last step, I am putting in an anonymous object. You can put the data in a "new ProductOrmEntity() { ProductID = pocoMaxUpdateDatePerParent.ProductID }...or you can get the FULL ProductOrmEntity object. Your original code, I don't know if getting all columns of the Product object is what you want, or only some of the columns of the object.

NHibernate QueryOver RowCount, differentiate between 0 and NULL

I'm trying to get all active tasks from an employee by QueryOver and RowCount with this code:
var id = 1;
var activeTasks = Session
.QueryOver<Employee>()
.Where(emp => emp.id == id)
.JoinQueryOver<Tasks>(emp => emp.Tasks, JoinType.InnerJoin)
.Where(task => task.State == TaskState.Active)
.RowCount();
If there is no employee with the specified id, RowCount() returns 0.
The thing is I need to know if the employee didn't exist or if there was no active tasks.
I could do this with 2 queries where I would first get the employee and check for null, then query for the tasks. But ideally if it's possible I would want this to go all in one query.

You can do it much simpler with LINQ to NHibernate:
var employeeWithActiveTaskCount = session
.Query<Employee>()
.Where(e => e.Id == id)
.GroupJoin(
session
.Query<Task>()
.Where(t => t.State == TaskState.Active),
e => e.Id,
t => t.Employee.Id,
(e, t) => new { Employee = e, Tasks = t })
.Select(et => new
{
EmployeeId = et.Employee.Id,
TaskCount = et.Tasks.Count()
});
This will return only existing employees with appropriate number of tasks. In case the employee doesn't exist, empty collection is returned. The query generated by this looks as follows:
select employee0_.Id as col_0_0_, (select cast(count(*) as INT)
from [Task] task1_ where task1_.State=?
and (task1_.Employee_id=employee0_.Id
or (task1_.Employee_id is null)
and (employee0_.Id is null)))
as col_1_0_ from [Employee] employee0_ where employee0_.Id=?

Null expression with dynamic LINQ

I have a method with the following signatures that return a C# expression
Expression<Func<T, bool>> GetExpression<T>(IList<Filter> filters)
Then the following code that uses Dynamic LINQ
using (TestContext tEntities = new TestContext())
{
var filterExp = Exp.ExpressionBuilder.GetExpression<Client>(filters);
var filteredCollection = tEntities.Client.Where(filterExp);
IQueryable<Client> queryResult;
if (filterExp == null)
queryResult = tEntities.Client;
else
queryResult = tEntities.Client.Where(filterExp);
}
This is a simple scenario.
I have queries that are 50 lines long sometimes more. I want to avoid having the same code twice with only difference using the where clause.
Does anyone knows if I achieve something different?
from product in context.Product.Where(deleg)
.Include(x => x.Type)
.Include(x => x.Category)
.Include(x => x.WareHouse)
.Include(x => x.Photos)
join f in context.Favorite on product.Id equals f.ProductFid into fg
from fgi in fg.Where(f => f.UserFid == userId).DefaultIfEmpty()
orderby product.Id descending
select new ProductngDto()
{
ProductItem = product,
FavoriteId = fgi != null ? fgi.Id : (long?)null
}).Skip(page * pageSize).Take(pageSize);
Thanks in advance

One of the nice things about IQueryable<T> and IEnumerable<T> is the fact that they are so abstract, you can easily chain them together. One solution could be to structure your code like this:
using (TestContext tEntities = new TestContext())
{
var filterExp = Exp.ExpressionBuilder.GetExpression<Client>(filters);
var filteredCollection = tEntities.Client.Where(filterExp);
IQueryable<Client> queryResult = tEntities.Client;
if (filterExp != null)
{
queryResult = queryResult.Where(filterExp);
}
//do something else with queryResult
}
This way you can continue using queryResult without having to know, or even care, about whether or not filterExp was applied.
For the second example, shuffling it around could look something like this:
var query = from p in context.Product
.Include(x => x.Type)
.Include(x => x.Category)
.Include(x => x.WareHouse)
.Include(x => x.Photos);
if (deleg != null)
{
query = query.Where(deleg);
}
query = from product in query
join f in context.Favorite on product.Id equals f.ProductFid into fg
from fgi in fg.Where(f => f.UserFid == userId).DefaultIfEmpty();
orderby product.Id descending
select new ProductngDto()
{
ProductItem = product,
FavoriteId = fgi != null ? fgi.Id : (long?)null
}).Skip(page * pageSize).Take(pageSize);
Another option you have, is to check the filterExp for null and assign an "always true" lambda to it.
If you control the GetExpression<T> method, you could add it in there as a last step. If you don't control it, you could do the null checking in the methods where you use it's result.
This will allow you to keep your code looking a bit cleaner, but the trade-off is a small performance hit for having to evaluate the lambda all the time.
Depending on how your expressions are typed, this might be as easy as:
if (filterExp == null)
{
filterExp = (_) => true;
}

Entity Framework filter nested collection

I have a entity relation diagram as follows.
ClassEntity:
public int id
public int std
public virtual ICollection<StudentEntity> students
StudentEntity:
public int id
public string name
public string gender
public virtual ClassEntity class
public virtual StudentAddressEntity studentAddress
StudentAddressEntity:
public int id
public string address
I need to get the class and its male children.
var classEntity = dbContext.Set<ClassEntity>().Where(t => t.id == classId);
var query = classEntity.Include(c => c.students.Select(s => s.studentAddress))
.FirstOrDefault(c => c.students.Any(s => s.gender == GenderEnum.Male));
But it is returning the class with all the students. How to filter only male students?

I have used joins to accomplish similar results in the past. For eg I've accounts that have addresses nested (1:M). If I want to get, say, all the accounts that belong to a particular country, I would use joins as below:
(from a in accountRepo.GetAll()
join aa in accountAddressRepo.GetAll() on a.AccountId equals aa.AccountId
join ad in addressRepo.GetAll() on aa.AddressId equals ad.AddressId
where ad.CountryId == codeCountryId
select a).ToList();
If you are not using repository pattern you can simply replace accountRepo.GetAll() with DbContext.Set().
In your case you should be able to join Student, Address and Class entities and get similar results. Something like below should work for you:
(from s in DbContext.Set<StudentEntity>
join a in DbContext.Set<StudentAddressEntity> on s.studentAddress.id equals a.id
join c in DbContext.Set<ClassEntity> on s.class.id equals c.id
where c.std == classId && s.gender== GenderEnum.Male
select s).ToList();
please note this is a simple representation based on my understanding of your database and entity names. You may need to tweak this query a bit to make it compilable but the underlying idea should work for you. Please let me know how did it work for you.

You intentionally "can't" do this directly with the EF proxies. For example, consider what would happen when you tried to call SaveChanges() and all of the female students are missing from ClassEntity.Students!
Instead, the usual thing to do if you're just displaying data is to project onto an anonymous type or a DTO, e.g.:
var classOnlyMale = dbContext.Set<ClassEntity>()
.Where(x => x.Id == classId)
.Select(x => new // I'm using an anonymous type here, but you can (and usually should!) project onto a DTO instead
{
// It's usually best to only get the data you actually need!
Id = x.Id
Students = x.Students
.Where(y => y.Gender == GenderEnum.Male)
.Select(y => new { Name = y.Name, ... })
});
Or, if you desperately need to make changes and save them:
var classOnlyMale = dbContext.Set<ClassEntity>()
.Where(x => x.Id == classId)
.Select(x => new
{
Class = x,
MaleStudents = x.Students.Where(y => y.Gender == GenderEnum.Male)
});
I quite strongly recommend the former unless there's no way around it. It's really easy to introduce bugs if you're making changes to filtered data and trying to save it.

The below should load only the male students for each class.
var classEntity = testContext.Set<ClassEntity>().Where(t => t.Id == classId);
var classes = classEntity.ToList().Select(c =>
{
testContext.Entry(c)
.Collection(p => p.Students)
.Query()
.Where(s => s.Gender == GenderEnum.Male)
.Load();
return c;
});

I think join is the right way to go about it as suggested by Manish Kumar.
(from s in DbContext.Set<StudentEntity>
join a in DbContext.Set<StudentAddressEntity> on s.studentAddress.id equals a.id
join c in DbContext.Set<ClassEntity> on s.class.id equals c.id
where c.std == classId && s.gender== GenderEnum.Male
select s).ToList();

C# Retrieve Common Records by LINQ

I have a database with the table that keeps user_ids and tag_ids. I want to write a function which takes two user_ids and returns the tag_ids that both users have in common.
These are the sample rows from the database:
User_id Tag_id
1 100
1 101
2 100
3 100
3 101
3 102
What I want from my function is that when I call my function like getCommonTagIDs(1, 3), it should return (100,101). What I did so far is that I keep the rows which are related to user_id in two different lists and then using for loops, return the common tag_ids.
using (TwitterDataContext database = TwitterDataContext.CreateTwitterDataContextWithNoLock())
{
IEnumerable<Usr_Tag> tags_1 = database.Usr_Tags.Where(u => u.User_id == userID1).ToList();
IEnumerable<Usr_Tag> tags_2 = database.Usr_Tags.Where(u => u.User_id == userID2).ToList();
foreach (var x in tags_1)
{
foreach (var y in tags_2) {
if (x.Tag_id == y.Tag_id) {
var a =database.Hashtags.Where(u => u.Tag_id==x.Tag_id).SingleOrDefault();
Console.WriteLine(a.Tag_val);
}
}
}
}
What I want to ask is that, instead of getting all rows from database and searching for the common tag_ids in the function, I want to get the common tag_ids directly from database with LINQ by making the calculations on the database side. I would be grateful if you could help me.
This is the SQL that I wrote:
SELECT [Tag_id]
FROM [BitirME].[dbo].[User_Tag]
WHERE USER_ID = '1' AND Tag_id IN (
SELECT [Tag_id]
FROM [BitirME].[dbo].[User_Tag]
where USER_ID = '3')

What you want is the "Intersection" of those two sets:
var commonTags = database.Usr_Tags.Where(u => u.User_id == userID1).Select(u => u.Tag_id)
.Intersect(database.Usr_Tags.Where(u => u.User_id == userID2).Select(u => u.Tag_id));
And voila, you're done.
Or, to clean it up a bit:
public static IQueryable<int> GetUserTags(int userId)
{
return database.Usr_Tags
.Where(u => u.User_id == userId)
.Select(u => u.Tag_id);
}
var commonTags = GetUserTags(userID1).Intersect(GetUserTags(userID2));

Here's one way to do it:
int[] users = new int[] {1,3}; // for testing
database.Ustr_Tags.Where(t => users.Contains(t.User_id))
.GroupBy(t => t.Tag_id)
.Where(g => users.All(u => g.Any(gg=>gg.User_id == u))) // all tags where all required users are tagged
.Select(g => g.Key);
One benefit of this one is it can be used for any number of users (not just 2).

If i got it right, query like this is maybe what you need
var q = from t in database.Usr_Tags
//all Usr_Tags for UserID1
where t.User_Id == userID1 &&
//and there is a User_tag for User_ID2 with same Tag_ID
database.User_Tags.Any(t2=>t2.User_ID==userID2 && t2.Tag_ID==t.Tag_ID)
select t.Tag_Id;
var commonTags = q.ToList();