EF Core with GroupBy - column ambiguously defined - oracle

Question

Trying to filter and group by an IQueryable I get an oracle exception "column ambiguously defined".
The code looks as follows:
public class UserRepository{
public IEnumerable<UserDto> GetUsers(UserFilter filter){
IQueryable<User> users = _context.Users;
return filter.Set(users)
.Select(user => new UserDto(){
Id = user.Id,
NoOfTweets = user.NoOfTweets
}
.ToListAsync();
}
}
public class UserFilter : Filter{
public string? Name {get; set;}
public UserGroupBy? GroupBy{get; set;} //Enum
public override IQueryable<Model> Set(IQueryable<Model> models){
IQueryable<User> users = models.Cast<User>();
users = !string.IsNullOrEmpty(Name)
? users.Where(user => user.Name.Equals(Name))
: users;
if(GroupBy.HasValue)
{
users = GroupBy.Value switch{
UserGroupBy.Name => users
.GroupBy(user => new{
user.Name,
user.Birthday
})
.Select(group => new User(){
Name = group.Key.Name,
NoOfTweets = group.Sum(e => e.NoOfTweets)
})
_ => throw new NotImplementedException()
}
}
return base.Set(users); // in base class the list gets paginated
}
}
I guess the error is because of the two selects (in group-by and finally into userGet) but how can I workaround that cleanly?
I hoped that the entity Framework solves this using aliases, but it doesn't.
My Versions:
Oracle.EntityFrameworkCore: 6.21.61
.Net 6
Additional Thoughts
Doing the Group by after the paginate leads to an incorrect result because the wrong list gets paginated.
It seems that the second Select reexecutes the first on so that the group.Sum(...) is called twice

Answer 1

I know nothing about EF Core, but - as far as Oracle is concerned & based on what you said so far, there are (at least) two positions that might raise the error. I'll try to illustrate it on a simple query which selects sum of salaries per each department.
Asterisk points to position which raised the error:
SQL> select deptno, sum(sal)
2 from emp join dept on emp.deptno = dept.deptno
3 group by deptno;
group by deptno
*
ERROR at line 3:
ORA-00918: column ambiguously defined
OK, so let's add table name - in vain, it still doesn't work because column in select column list also misses its source table name:
SQL> select deptno, sum(sal)
2 from emp join dept on emp.deptno = dept.deptno
3 group by dept.deptno;
select deptno, sum(sal)
*
ERROR at line 1:
ORA-00918: column ambiguously defined
Finally, when fixed:
SQL> select dept.deptno, sum(sal)
2 from emp join dept on emp.deptno = dept.deptno
3 group by dept.deptno;
DEPTNO SUM(SAL)
---------- ----------
30 9400
20 10875
10 8750
SQL>
As I said: I can't interpret code you wrote and don't see anything like what I posted, but I hope that these examples might help you find the culprit.