Being new to LINQ, I created a couple queries and would like to combine them into one, but I am not sure how to do it. Here they are:
var u = dc.Users.Where(w => w.UserName == userName).SingleOrDefault();
var m = dc.Memberships.Where(w => w.UserId == u.UserId).SingleOrDefault();
m.PasswordQuestion = securityQuestion;
m.PasswordAnswer = securityAnswer;
dc.SubmitChanges();
dc.Users is the aspnet_Users table
dc.Membership is the aspnet_Membership table
Also, What is the difference between SingleOrDefault and FirstOrDefault?
Not sure if they have a relationship (they should). If they do, then the linq-to-sql designer will give you a User property on the membership object (or the other way around). Then you can write something like this:
var membership = dc.Memberships.Where(x => x.User.UserName == userName).SingleOrDefault();
If they don't have a relationship you can write something like this:
var membership = (from m in dc.Membership join u in dc.Users on u.UserId equals m.UserId
where u.UserName == userName
select u).SingleOrDefault();
The difference between SingleOrDefault() and FirstOrDefault() is that SingleOrDefault() assumes that there is no more then one item that matches the query. If two items match the query then a exception will be thrown. While if you use FirstOrDefault() and there is two items that match the query then the first one will be selected.
SingleOrDefault means "if there are no elements, give me the default, if there is one, give me it, otherwise, throw an exception". FirstOrDefault means "if there are no elements then give me the default, otherwise give me the first one".
As for your question about combining the queries -- why would you? If the code is working well as written, why change it?
To answer a question you didn't ask: the usual way of combining queries is to use a query continuation. A common pattern is:
var q1 = from y in something
somequeryclauses;
var q2 = from x in q1
someotherquerqyclauses;
You could write this as one big query like this:
var q2 = from x in (from y in something
somequeryclauses)
someotherqueryclauses;
which gets hard to read. You can combine the two queries with a query continuation:
var q2 = from y in something
somequeryclauses
into x
someotherqueryclauses;
Make sense?
SingleOrDefault will throw an error if your sequence contains more than one element, FirstOrDefault does not. In this case, you'd probably want FirstOrDefault, because users can have more than one membership.
var m = dc.MemberShips where(w => w.UserId.UserName == userName).FirstOrDefault();
I don't have Visual Studio in front of me, but it would be something like this:
var m = dc.Memberships.Where(w => w.Users.UserName == userName).SingleOrDefault();
m.PasswordQuestion = securityQuestion;
m.PasswordAnswer = securityAnswer;
dc.SubmitChanges();
w.Users allows you to follow the foreign key link between the membership and users tables.
SingleOrDefault will throw an exception if more than one result is returned. FirstOrDefault returns the first record only.
Related
I am trying to write a LINQ query equivalent to below SQL
SELECT DISTINCT m.*,rm.RoleId FROM dbo.Menu m
INNER JOIN dbo.RoleMenu rm on m.Id=rm.MenuId
INNER JOIN dbo.RoleUser ru on rm.RoleId=ru.RoleId
WHERE ru.UserName='dd#dd.com' and m.Url='/dashboard#/pm'
I came with the below query which is not returning the expected output
var auth = _context.RoleUsers.Where(
x => x.Role.MenuRoles.FirstOrDefault().Menu.Url == pagePermissions.Url
&& x.UserName == pagePermissions.UserName).Count()
May I know a better way to do this?
Your sql looks at all the menus related to a role user, but your Linq is only looking at the first one. I think you want x.Role.MenuRoles.Any(mr => mr.Menu.Url == pagePermissions.Url). But then you're also doing a Count on the matching users instead of selecting the menus that match that url. A closer translation would be.
var results = (from m in _context.Menus
from rm in m.RoleMenus
from ru in rm.RoleUsers
where m.Url == pagePermissions.Url
&& u.UserName == pagePermissions.UserName
select new { Menu = m, rm.RoleId }).Distinct();
You may have to adjust some of the navigation properties as I was just guessing at them. They usually are pluralizations of the tables, but I see in your Linq that you have MenuRoles instead of RoleMenus.
Suppose I have a list of {City, State}. It originally came from the database, and I have LocationID, but by now I loaded it into memory. Suppose I also have a table of fast food restaurants that has City and State as part of the record. I need to get a list of establishments that match city and state.
NOTE: I try to describe a simplified scenario; my business domain is completely different.
I came up with the following LINQ solution:
var establishments = from r in restaurants
from l in locations
where l.LocationId == id &&
l.City == r.City &&
l.State == r.State
select r
and I feel there must be something better. For starters, I already have City/State in memory - so to go back to the database only to have a join seems very inefficient. I am looking for some way to say {r.City, r.State} match Any(MyList) where MyList is my collection of City/State.
UPDATE
I tried to update based on suggestion below:
List<CityState> myCityStates = ...;
var establishments =
from r in restaurants
join l in myCityStates
on new { r.City, r.State } equals new { l.City, l.State } into gls
select r;
and I got the following compile error:
Error CS1941 The type of one of the expressions in the join clause is incorrect. Type inference failed in the call to 'Join'.
UPDATE 2
Compiler didn't like anonymous class in the join. I made it explicit and it stopped complaining. I'll see if it actually works in the morning...
It seems to me that you need this:
var establishments =
from r in restaurants
join l in locations.Where(x => x.LocationId == id)
on new { r.City, r.State } equals new { l.City, l.State } into gls
select r;
Well, there isn't a lot more that you can do, as long as you rely on a table lookup, the only thing you can do to speed up things is to put an index on City and State.
The linq statement has to translate into a valid SQL Statement, where "Any" would translate to something like :
SELECT * FROM Restaurants where City in ('...all cities')
I dont know if other ORM's give better performance for these types of scenarios that EF, but it might be worth investigating. EF has never had a rumor for being fast on reads.
Edit: You can also do this:
List<string> names = new List { "John", "Max", "Pete" };
bool has = customers.Any(cus => names.Contains(cus.FirstName));
this will produce the necessary IN('value1', 'value2' ...) functionality that you were looking for
Because of a poor design on our database I have to write a complex query to get my data.
I need to get all valid data from a table_1. In other works I need to get each valid row of my table_1. I don't have a simple valid or invalid column in my table_1. This information is stored in a table_2. Table_2 contains all invalid row with the error message.
Let say the data I need to retrieve are orders. For my example notice that OrderNo is the number of an order. So I can have multiple line, version, of this order in table_1 and I can also have multiple line of error on this order in table_2. So I will also have to use a version number.
I already tried this:
table_1.Where(y => (y.OrderNo == "1234"));
table_2.Where(y => (y.OrderNo == "1234")).Select(y => y.Version).Distinct();
And I think I need to do something like this:
var errorList = table_2.Where(y => (y.OrderNo == "1234")).Select(y => y.Version).Distinct();
table_1.Where(y => (y.OrderNo == "1234" && y.Version.NOT_IN(erriList)));
Could you help me?
I suppose you are searching for Contains function with ! symbol (logical negation operator). Like this:
var errorList = table_2.Where(y => y.OrderNo == "1234")
.Select(y => y.Version);
var res = table_1.Where(y => y.OrderNo == "1234"
//here you get only rows that doesn't represent in errorList
&& !errorList.Contains(y.Version));
to get data from a table but not if in another table
This is called antijoin. While you can use Contains and Any based approaches presented in the other answers, usually you'll get the best performance by using the classic SQL approach - LEFT OUTER JOIN combined with checking the right side for NULL.
Which in LINQ looks like this:
var query =
from t1 in table_1
//where t1.OrderNo == "1234"
join t2 in table_2 on t1.OrderNo equals t2.OrderNo into t2group
from t2 in t2group.DefaultIfEmpty()
where t2 == null
select t1;
Actually when you use OrderNo filter, most probably there will not be a noticeable speed difference between this and other queries. The main benefit of the above would be if you remove that filter, although many nowadays SQL query optimizers are able to derive one and the same execution plan regardless of whether the query uses JOIN / IN / EXISTS constructs.
How about this:
var validRows = table1
.Where(t1 => !table2
.Any(t2 => t1.OrderNo == t2.OrderNo &&
t1.Version == t2.Version));
Note that this is far more efficient in SQL unless you're using something fancy that translates the expression to SQL.
I want to know how to use IN clause in Linq. Here is my Code :-
int empCount = ctx.tblEmpTran
.Count(
e => e.Id == Id &&
e.Month == selectedMonth &&
e.Year == selectedYear &&
e.employeeId.contains()
);
The Following query is supposed to be in IN
SELECT A.Id FROM dbo.EmployeeDetail A WHERE A.CompanyId = 1 AND A.COLS > 0
In the above code, contains method do not popup in intellisense.
This is because you are trying to convert from SQL to Linq, and you couldn't try a worse approach.
You should try to write your LINQ query starting from what you need it to do, forgetting SQL altogether.
In Linq there is no "IN" operator, to achieve the same thing you take your collection and check if it Contains the value.
So in your scenario you should simply generate your collection of valid values and then in your query do:
myCollection.Contains(e.employeeId)
It is "collection CONTAINS value" the logic, not "value is found IN collection". Again if you insist to start from SQL when using Linq you will always incur in this kind of problems.
Check Albahari tutorial on how to approach Linq correctly and your productivity will skyrocket.
You should create a collection of employee IDs that you want to check, and the code would be
employees.contains(e.employeeId)
Instead of this e.employeeId.contains(), you should use this:
listOfIds.Contains(e.employeeId)
where listOfIds would be a list of int, List<int> and would contain the ids you would place between the parentheses after IN(...).
considering that you have a tblEmployeeDetail in the same DbSet and them having a relation through employeeId you can write you query like.
var q = from e in ctx.tblEmployeeDetail where e.Transactions.Any(t => t.Month == selectedMonth &&
t.Year == selectedYear);
int empCount = q.Count();
this is pseudo-code but this is how you would use the LINQ effectively, (Exists is better than In check)
I'm having trouble coming up with an efficient LINQ-to-SQL query. I am attempting to do something like this:
from x in Items
select new
{
Name = x.Name
TypeARelated = from r in x.Related
where r.Type == "A"
select r
}
As you might expect, it produces a single query from the "Items" table, with a left join on the "Related" table. Now if I add another few similar lines...
from x in Items
select new
{
Name = x.Name
TypeARelated = from r in x.Related
where r.Type == "A"
select r,
TypeBRelated = from r in x.Related
where r.Type == "B"
select r
}
The result is that a similar query to the first attempt is run, followed by an individual query to the "Related" table for each record in "Items". Is there a way to wrap this all up in a single query? What would be the cause of this? Thanks in advance for any help you can provide.
The above query if written directly in SQL would be written like so (pseudo-code):
SELECT
X.NAME AS NAME,
(CASE R.TYPE WHEN A THEN R ELSE NULL) AS TypeARelated,
(CASE R.TYPE WHEN B THEN R ELSE NULL) AS TypeBRelated
FROM Items AS X
JOIN Related AS R ON <some field>
However, linq-to-sql is not as efficient, from your explanation, it does one join, then goes to individually compare each record. A better way would be to use two linq queries similar to your first example, which would generate two SQL queries. Then use the result of the two linq queries and join them, which would not generate any SQL statement. This method would limit the number of queries executed in SQL to 2.
If the number of conditions i.e. r.Type == "A" etc., are going to increase over time, or different conditions are going to be added, you're better off using a stored procedure, which would be one SQL query at all times.
Hasanain
You can use eager loading to do a single join on the server to see if that helps. Give this a try.
using (MyDataContext context = new MyDataContext())
{
DataLoadOptions options = new DataLoadOptions();
options.LoadWith<Item>(i => i.Related);
context.LoadOptions = options;
// Do your query now.
}