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Fetching only one column from a table via where and select LINQ statement

I have written two queries which look like:
var loggedUser2 = ctx.Users.Where(y => y.Email == User.Identity.Name).Select(usr => new Users { UserId = usr.UserId }).AsEnumerable();
And second query looks like this:
var loggedUser = ctx.Users.FirstOrDefault(y => y.Email == User.Identity.Name);
The second query noticabely takes much more time to pull a single record from a table from a remote server as I can tell.
For the first query I'm getting an error:
The entity or complex type 'Users' cannot be constructed in a LINQ to Entities query.
when I try to access the property UserId of the "Users" object.
Now I have a couple of questions:
Why do I get this error and how do I actually access a property of a collection which is IEnumerable? I get it that I can materialize the query by using .ToList() and then have it accessed at [0].UserId, but I don't think this is the right way to do it?
What is the most efficient way to select a single column for a single records from a table (UserId in my case)? How would that query looks like (besides using a stored procedure).
Can someone help me out ? :)

you can declare a DTO or you can use an anonymous type like this:
var user = ctx.Users
.Where(u => u.Email == email)
.Select(u => new { UserId = u.UserId })
.FirstOrDefault()

linq: aggregate into single line (flatten)

I am currently using a linq and accessing some meta data using a for loop:
public SignUpMeta GetSignUpMeta(User user)
{
var users = (from u in User
where user.Email == u.Email || user.UserName == u.UserName
select u);
var result = new SignUpMeta();
foreach (var u in users)
{
if (user.Email == u.Email) result.IsDuplicateEmail = true;
if (user.UserName == u.UserName) result.IsDuplicateUserName = true;
}
return result;
}
Is there any way for the Linq to generate the SignUpMeta directly?
This function is inside the DBContext class and I am looking for a way to get the exact data directly from the db (without raw sql).
Update:
The User is DbSet and the whole code runs inside the db context class. I am trying to write something that will make EF fetch the value directly in a single query.
Update 2:
The SQL equivalent of what I am looking for would be:
SELECT MAX(username), MAX(email)
(SELECT CAST((UserName = #user) AS bit) username,
CAST((Email = #email) AS bit) email
FROM User WHERE UserName = #user OR Email = #email)
Update 3:
SignUpMeta object that needs to be fetched contains the metadata that provides information required for server side validation.
The above C# code runs a query that fetches up to two columns in this instance. When there are more such conditions, there would be more lines. I am trying to find a way that EF would give only the two booleans alone in a single record.

This'll be my try, if you truly must use LINQ:
from u in stuff.Users
group u by 0 into grp
select new
{
IsDuplicateEmail = grp.Any(x => x.Email == user.Email),
IsDuplicateUserName = grp.Any(x => x.UserName == user.UserName)
}
Entity Framework will translate that into sub-selects. If you're using SQL Server and have both columns indexed, this should result in the same amount of I/O as your sample SQL.
I don't believe there is any query that will generate your desired sample SQL.

I think this will be the fastest query:
public SignUpMeta GetSignUpMeta(User user)
{
return new SignUpMeta()
{
IsDuplicateEmail = User.Where(u => u.Email == user.Email).Take(1).Any(),
IsDuplicateUserName = User.Where(u => u.UserName == user.UserName).Take(1).Any(),
};
}
Caching on the DB server should make the two queries quite fast.

How to add condition to LINQ query based on Id

This is my action method which fetches all the users with their Id.
public JsonResult GetUsers()
{
var ret = (from user in db.Users.ToList()
select new
{
UserName = user.UserName,
// i am stuck here, i want to get all those ids whom current logged user is following
Idfollowing = user.FollowTables.Contains()
Idnotfollowing =
});
return Json(ret, JsonRequestBehavior.AllowGet);
}
the structure of FollowTable is like this:
ID UserId FollowId
1 4 11
2 4 12
2 4 13
here, current loggedin user's id is 4 and he is following 11, 12, 13 so i want to return only 11, 12 and 13 to Idfollowing and rest remaining id in the Idnotfollowing. how to get it done.
Well, i think with list or array, i will not get desired result. so, i want to add something here.
Well, with every UserName an id is also passed to view page. So, i have break them into two.Now, how to assign values to these ids.
Comapre User.Id with Current loggedin user's follow table's followId column.If match is found .i.e if id matches or found then assign that user.Id to Idfollowing and null to Idnotfollowing and vice versa in opposite case.
I have to generate follow unfollow button based on these ids returned.
public JsonResult GetUsers()
{
int currentUserId = this.User.Identity.GetUserId<int>();
var ret = (from user in db.Users.ToList()
let Id = user.FollowTables.Where(x => x.UserId == currentUserId).Select(f => f.FollowId).ToList()
let Idnot = (from user2 in db.Users
where !Id.Contains(user2.Id)
select user2.Id).ToList()
select new
{
UserName = user.UserName,
Id = Id,
//Id = user.FollowTables.Where(x => x.UserId == currentUserId)
// .Select(x => x.FollowId).Single(),
Idnot = Idnot,

It looks like you have a standard one-to-many relationship from User to FollowTable. This data model enforces that user.FollowTables only contains followers. You won't be able to fill in Idnotfollowing from the FollowTables property directly.
Something like this may work:
var query = (
from user in db.Users // note: removed ToList() here
// to avoid premature query materialization
where //TODO ADD WHERE CLAUSE HERE ?
let followIds = user.FollowTables.Select(f => f.FollowId)
let notFollowIds = (from user2 in db.Users
where !followIds.Contains(user2.Id)
select user2.Id)
select new
{
UserName = user.UserName,
Idfollowing = followIds.ToArray(),
Idnotfollowing = notFollowIds.ToArray()
})
// TODO add paging? .Skip(offset).Take(pageSize)
.ToList();
Do verify the SQL generated by this query and make sure it performs ok though...
Also, note that I removed the .ToList() from db.Users.ToList() to avoid premature query materialization. It is generally a bad idea anyway to extract all data from a table unconstrained, you will typically want to a

var ret = (from user in db.Users.ToList()
select new
{
UserName = user.UserName,
Idfollowing = user.FollowTables.Select(x=> x.Id)
Idnotfollowing = db.FollowTables.Where(x=> !user.FollowTables.Select(x=> x.Id).Contains(x.Id)).Select(x=> x.Id)
});
it's ugly but will work, there must be another better way to do.

You can simply use a Where method to filter the table and use Select to project FollowiId:-
var ret = (from user in db.Users.ToList()
select new
{
UserName = user.UserName,
Idfollowing = user.FollowTables.Where(x => x.UserId == user.Id)
.Select(x => x.FollowId).ToArray(),
Idnotfollowing = user.FollowTables.Where(x => x.UserId != user.Id)
.Select(x => x.FollowId).ToArray()
});
Assuming, Idfollowing & Idnotfollowing are array if integers (if FollowId is integer) otherwise you can replace it with ToList if its a list instead.

How can i do it using linq2sql...?

I have a table Users. Users has a column rating. How i can get information about user place using linq2sql? I want method like:
var userPlace =
GetUserPlaceById(userId);
Table Users may contains a few thousands users.
Sorry guys. Users DOESNT contain place column. Real example: Rating is chess elo rating. if you have high rating then you on 1st place. If you have lower rating then you on the last place.

Did you mean something like this?
int userRating = users.Single(user => user.Id = userId).Rating;
int userPlace = users.Where(user => user.Rating < userRating).Count() + 1;

I have a table Users. Users has a column rating. How i can get information about user place using linq2sql?
I'm not sure what "userPlace" is, but assuming it is a column in that table...
var userPlace = (from user in db.Users
where user.Id == userId
select user)
.First()
.UserPlace;
Be aware that calling .First() will throw an exception if no match is found, so if you expect that sometimes this user will not exist use FirstOrDefault, check for null, and then grab the UserPlace property.

You would use something like:
string GetUserPlaceById(int userId)
{
IQueryable<User> users = GetUsers(); // Get users queryable reference
return users.Single(user => user.Id == userId).Place;
}

You could do something like this:
var userPlace = _db.Users.Where(x => x.UserId == userId).Select(x => x.Place).SingleOrDefault();

Making a LINQ query better

I'm having a hard time getting the LINQ-syntax.. How can I do this command in a better way?
var user = (from u in context.users
where u.email.Equals(email)
select u).Single();
var pinToUser = (from ptu in context.pintousers
where ptu.user_id.Equals(user.id)
select ptu).Single();
var pin = (from p in context.pins
where p.idpin.Equals(pinToUser.pin_idpin)
select p).Single();
return pin;
As you can see, there's a table user, a table pintouser and a table pin. Pintouser references user and pin. Is it possible to write something short like "user.pintouser.pin"? I think I have the navigation properties all set up but I'm not sure how to use them properly or if I could make them better by modifying them.
Thanks for reading

Use joins to rewrite everything as a single clean query. If I read your queries properly, this should give you the correct result:
var pin = (from u in context.users
join ptu in context.pintousers on u.id equals ptu.user_id
join p in context.pins on ptu.pin_idpin equals p.idpin
where u.email == email
select p).Single();
Keep in mind, though, that if this query returns anything other than a single result your code will throw an Exception.
If you want to handle the possibility of getting one or no rows then you should use SingleOrDefault().
If you want to handle the possiblity of getting any number of rows then you should really use FirstOrDefault().

Note that if you have your foreign-key relationship set righ in your database, Linq-to-Sql should have the joins for you automatically:
var pin = (from u in context.users
where u.email == email
select u.pintouser.pin).Single();
which means you can reduce this to:
var pin = context.users.Where(u=>u.email == email)
.Select(u=>u.pintouser.pin)
.Single();
(UPDATE Note: I had originally suggested the following, which is much shorter, but I believe it will cause two round-trips to the database)
var pin = context.users.Single(u=>u.email == email).Single().pintouser.pin;
Now, the .pintouser.pin is safe, because the Single() will always return a user object (or throw an exception).

You should be using join, as #JustinNiessner points out, but this is another way to write your query.
var user = context.users.Single(u => u.email == email);
var pinToUser = context.pintousers.Single(ptu => ptu.user_id == user.id);
var pin = context.pins.Single(p => p.idpin == pinToUser.pin_idpid);

Since you have navigation properties, might as well use them:
Pin pin =
(
from u in context.Users
where u.email == email
from ptu in u.pintousers
let p = ptu.pin
select p
).Single();