Develop Reference

net core 5.0 .FirstOrDefault()' could not be translated

users = (List<UserResponse>)
(from u in _context.Users
join rolePermiss in _context.RolePermissions on u.RoleId equals rolePermiss.RoleId
group new { u, rolePermiss} by u.Id into gr
select new UserResponse()
{
Id = gr.FirstOrDefault().u.Id,
//Permissions = gr.FirstOrDefault().rolePermiss.Select(x => x.Id).ToList()
}).ToList();

try this linq
users = (List<UserResponse>)
(from u in _context.Users
join rolePermiss in _context.RolePermissions on u.RoleId equals rolePermiss.RoleId
group new { u, rolePermiss } by u.Id into gr
select gr.FirstOrDefault() into mr
select new UserResponse()
{
Id = mr.u.Id,
//............
}).ToList();

iqueryable with sub query as outer join

I have a sample to look into Async calls and i need to get a count from the sub query. I know how to write this in a TSQL query but i am bit confused with the iqueryable use.
Here is what i currently have. I am getting the users and then get the count inside a loop. How can i do the loop part in the first query?
public static async Task GetUsers(this List<UserViewModel> users)
{
var db = ApplicationDbContext.Create();
users.AddRange(await (from u in db.Users
select new UserViewModel
{
Id = u.Id,
Email = u.Email,
FirstName = u.FirstName,
LastName = u.LastName
}).OrderBy(o => o.Email).ToListAsync());
if (users.Any())
{
foreach(var user in users)
{
user.SubscriptionsCount = await (from us in db.UserSubscriptions
join s in db.Subscriptions on us.SubscriptionId equals s.Id
where us.UserId.Equals(user.Id)
select us).CountAsync();
}
}
}

Could be handled in one of the following two ways. I have picked #2 for my sample.
1 : with sub query
var singleQuery = from u in db.Users
join sub in (from us in db.UserSubscriptions
join s in db.Subscriptions on us.SubscriptionId equals s.Id
group us by us.UserId into countGroup
select new { Count = countGroup.Count(), UserId = countGroup.Key })
on u.Id equals sub.UserId into sub1
from subR in sub1.DefaultIfEmpty()
select new UserViewModel
{
Id = u.Id,
Email = u.Email,
FirstName = u.FirstName,
LastName = u.LastName,
SubscriptionsCount = subR.Count == null ? 0 : subR.Count
};
var siteUsersSub = await (query).OrderBy(o => o.Email).ToListAsync();
2: Composing from sub queries
var subQuery = from us in db.UserSubscriptions
join s in db.Subscriptions on us.SubscriptionId equals s.Id
group us by us.UserId into countGroup
select new { Count = countGroup.Count(), UserId = countGroup.Key };
var query = from u in db.Users
join sq in subQuery on u.Id equals sq.UserId into sq1
from sqR in sq1.DefaultIfEmpty()
select new UserViewModel()
{
Id = u.Id,
Email = u.Email,
FirstName = u.FirstName,
LastName = u.LastName,
SubscriptionsCount = sqR.Count == null ? 0 : sqR.Count
};
var siteUsers = await(query).OrderBy(o => o.Email).ToListAsync();

sql query to c# using northwind DB

using the northwind DB, i have to make a query to get employeename, amount of orders per employee and average price of those orders
this is what the query looks like in SQL
SELECT TOP 10
a.LastName, a.FirstName, amountOfOrders = COUNT(DISTINCT b.OrderID), AveragePricePerOrder = SUM(c.Quantity*c.UnitPrice) /COUNT(DISTINCT b.OrderID)
FROM Employees a join orders b on (a.EmployeeID = b.EmployeeID)
join [Order Details] c on b.OrderID = c.OrderID
Group BY a.EmployeeID, a.LastName, a.FirstName
ORDER BY amountOfOrders Desc
this runs fine but I have to make this in c# and I am a little stuck
So far, I have got this
var query_rx = (from c in ctx.Employees
join or in ctx.Orders on c.EmployeeID equals or.EmployeeID
join ord in ctx.Order_Details on or.OrderID equals ord.OrderID
group c by new
{
c.EmployeeID,
c.LastName,
c.FirstName,
amount = c.Orders.Count
} into c
orderby c.Key.amount descending
select new
{
c.Key.LastName,
c.Key.FirstName,
amountOfOrders = c.Key.amount
}).Take(10);
"edit" I am having trouble working the average in, tried a lot of things but I can't get it to work
"edit" I have changed the query a bit with help from Dohnal's suggestion.
This looks almost exactly like what i want in terms of columns, except that the field lastname and firstname are blank, even with ToString
var query_rx = (from or in ctx.Order_Details
join ord in ctx.Orders on or.OrderID equals ord.OrderID
group or by new
{
ord.EmployeeID
} into c
orderby c.Select(x => x.OrderID).Distinct().Count() descending
select new
{
Lastname = (from emp in ctx.Employees
where c.Key.EmployeeID == emp.EmployeeID
select emp.LastName),
Firstname = (from emp in ctx.Employees
where c.Key.EmployeeID == emp.EmployeeID
select emp.FirstName),
c.Key.EmployeeID,
AmountOfOrders = c.Select(x => x.OrderID).Distinct().Count(),
AveragePricePerOrder = c.Sum(x => x.Quantity * x.UnitPrice) / c.Select(x => x.OrderID).Distinct().Count()
}).Take(10);

Try this query:
var query = (from emp in ctx.Employers
join order in ctx.Orders on emp.EmployeeID equals order.EmployerID
join orderDet in ctx.Order_Details on order.OrderID equals orderDet.OrderID
group new { emp, order, orderDet }
by new { emp.FirstName, emp.LastName, emp.EmployeeID, order.OrderID }
into orderGroup
let a = new
{
orderGroup.Key.EmployeeID,
orderGroup.Key.FirstName,
orderGroup.Key.LastName,
orderGroup.Key.OrderID,
sum1 = orderGroup.Sum(x => x.orderDet.Quantity * x.orderDet.UnitPrice),
}
group a by new { a.FirstName, a.LastName, a.EmployeeID } into empGroup
let a2 = new
{
empGroup.Key.FirstName,
empGroup.Key.LastName,
sum = empGroup.Sum(x => x.sum1),
count = empGroup.Count()
}
orderby a2.count descending
select new
{
a2.FirstName,
a2.LastName,
amountOfOrders = a2.count,
AveragePricePerOrder = a2.sum / a2.count
}).Take(10);

Check whether object NOT exists in another table in LINQ?

I have a query in entity framework which gets some messages from tables.
var ur = (from m in en.Messages
join mb in en.aspnet_Membership on m.FromUserId equals mb.UserId
join urs in en.UserProfiles on mb.UserId equals urs.UserId
join g in en.Groups on m.ToUserId equals g.GroupId
join ug in en.UserInGroups on g.GroupId equals ug.GroupId
where ug.UserId == userId
select new
{
InboxId = m.MessageId,
FromUser = urs.RaveName.Trim(),
CreatedOn = m.CreatedOn
}
).Concat(
// msg is not deleted
from m in en.Messages
join mb in en.aspnet_Membership on m.FromUserId equals mb.UserId
join urs in en.UserProfiles on mb.UserId equals urs.UserId
where m.ToUserId == userId
select new
{
InboxId = m.MessageId
,
FromUser = urs.RaveName.Trim()
,
CreatedOn = m.CreatedOn
}
);
I have another table which shows whether the message is used:
var msg = (from m in en.MessagesUsed
where m.UserId == userId
select m
);
Now, I need to check: Is there a message in 'ur' that is not in 'msg'? In T-SQL, we can use:
SELECT 1
FROM ur
WHERE NOT EXISTS (SELECT 1 FROM msg WHERE msg.Id = ur.InboxId AND msg.FromUser = ur.FromUser AND msg.CreatedOn = ur.CreatedOn)
to check this. But how to do it in LINQ?
Thanks

Entity Framework many-to-many conditional join

Right now I have 3 tables: User, Roles, and User_Roles for the many-to-many association. I want to create a search function that takes in a list of RoleIDs and return Users with that role. But if the RoleID list is empty, it should return all Users.
This is what I have so far..
public static List<User> SearchUser(List<int> roleIDs)
{
List<User> userList = new List<User>();
using (var db = new DBContext())
{
var users = (
from u in db.Users
join ur in db.User_Role
on u.UserID equals ur.UserID
join r in roleIDs
on ur.RoleID equals r
select u
).Distinct().ToList();
}
return userList;
}
but I can't figure out a way to conditionally join to User_Role only when roleIDs is not empty.
I also tried this but it didn't work.
var users = (from u in db.Users
from r in roleIDs
join ur in db.User_Role
on u.UserID equals ur.UserID
where roleIDs.Count == 0 || ur.RoleID == r
select u
).Distinct().ToList();
Is it possible to have a conditional join in Entity Framework?

Treat each case separately.
public static List<User> SearchUser(List<int> roleIDs)
{
using (var db = new DBContext())
{
if (roleIDs.Count == 0)
{
return db.Users.ToList();
}
var users = (
from u in db.Users
join ur in db.User_Role
on u.UserID equals ur.UserID
join r in roleIDs
on ur.RoleID equals r
select u
).Distinct().ToList();
return users;
}
}
You can simplify the join as follows if the join table is not mapped as separate entity.
public static List<User> SearchUser(List<int> roleIDs)
{
using (var db = new DBContext())
{
if (roleIDs.Count == 0)
{
return db.Users.ToList();
}
var users = db.Users.Where(u => u.Roles.Any(r => roleIDsContains(r.Id)))
.Distinct().ToList();
return users;
}
}

My biggest was concern keeping the code maintainable as new search criteria is added and to minimize the database calls. I ended up using if statements but didn't load it from the database till the very end. This resulted in one large sent query. I forced the database call by calling .ToList() at the end.
public static List<user> SearchUser(List<int> roleIDs, List<int> hobbyIDs)
{
List<User> Users = new List<User>();
IQueryable<User> tempUsers = null;
using (var db = new DBContent())
{
tempUsers = db.Users;
if (roleIDs.Count > 0)
{
tempUsers = from u in tempUsers
join ur in db.User_Role on u.UserID equals ur.UserID
join r in roleIDs on ur.RoleID equals r
select u;
}
if (hobbyIDs.Count > 0)
{
tempUsers = from u in tempUsers
join uh in db.User_Hobby on u.UserID equals uh.UserID
join h in hobbyIDs on uh.HobbyID equals h
select u;
}
//Go get it from the database
Users = ((ObjectQuery<User>)tempUsers)
.Include("Address").Distinct().ToList();
}
}