I'm looking for the most efficiant method to call the first and only the first item from a SQL Server database using Entityframework and linq.
I'm currently using
public static UserProfile GetUserProfile(Guid userID)
{
UserProfile oProfile = null;
using (var context = new MyEntities())
{
var profiles = from c in context.UserProfiles where c.UserID == userID select c;
if(profiles.Any())
{
oProfile = profiles.First();
}
}
return oProfile;
}
This however from what I can tell takes two DB ops to complete, one to check if the record exists and a second to return it. I'm sure there has to be a better pattern / method and I'm just not seeing it.
It's very simple: Use FirstOrDefault().
When there is no entry it will return null (like you have already), else it'll return the first entry. If you don't want your variable to take the null value, use a substitute in between.
edit:
Your code could be equally replaced with the following code, just that this one will just query once against the database.
public static UserProfile GetUserProfile(Guid userID)
{
UserProfile oProfile = null;
using (var context = new MyEntities())
{
oProfile = (from c in context.UserProfiles where c.UserID == userID select c).FirstOrDefault();
}
return oProfile;
}
If only one item should exist (as it looks in this case) then you should consider using Single(). Single will return the first item but throw an exception if more than one item exists - this can help maintain data quality if only a single item should exist.
If only one item should exist but it is optional then use SingleOrDefault which will act as Single but return null if no item exists.
You need to use First() or FirstOrDefault() without calling .Any()
var profile = (from c in context.UserProfiles where c.UserID == userID select c).First();
First() throws an exception if no records returned but FirstOrDefault() returns null in that case
Both will generate SQL like this:
SELECT TOP (1) ...
FROM ...
You should use the Take method:
Entity Framework/Linq to SQL: Skip & Take
It will only require one hit to the database.
Related
In my ASP.NET MVC application, when I'm going to fetch an item, I usually check its existence by using Any() then I will fetch that item by .Single().
However it comes to my mind to decrease number of database queries, and I tried to use this structure:
item myItem = null;
try
{
myItem = (from item in dbContext.items
where item.id == itemID
select item).Single();
}
catch (Exception e)
{
// i understand item is not exist in db
}
// if item is available
if (myItem ==null){
//item was not in db
}else{
//item is present id db
}
It only fetch database once, and if item does not exist it will throw exception so I do not need to check for Any() results any more.
Is this improve the efficiency of my application or it will cause some degradation due to exception handling?
No it is not. Why don't you use SingleOrDefault(). Then you won't need the exception handling for that situation and the remainder of the code can stay intact.
If you think there will be a value or a possible null, then use SingleOrDefault:
myItem = (from item in dbContext.items
where item.id == itemID
select item).SingleOrDefault();
Then check it:
// if item is available
if (myItem ==null){
//item was not in db
}else{
//item is present id db
}
As mentioned by other answers it is better to use .SingleOrDefault() and if you are using C# 6 you can use null conditional operator to simplify the expression.
myItem = (from item in dbContext.items
where item.id == itemID
select item).SingleOrDefault();
var somePropertyValue = myItem?.SomeProperty;
myItem?.SomeProperty will return value of the property or null if myItem is null.
Sorry I'm new to LINQ and Entity framework.
When a linq query fetches a whole Entity,
how can i return the result as an object type?
I mean not just a field but also to return all the fields of that specific record, as one entity.
For example a Staff table in sql. To return a staff object entity.
if i write Linq code like this:
var db= (from c in context.Staff
where c.id == sId
select new { s.sId,s.staffNo,s.name,s.surname,s.image});
return db; ---> here
Gives an error because i want to return result as Staff object.
How can it return as a table column object?
Thank you
As per your comments, you need a single object returned from your method.
To return a single Item you have following options.
First: Returns first item from the collection, if there are no matching items then an exception would be thrown.
FirstOrDefault: Works same like First except it would return null if there are no matching records.
Single: Returns only one item from the list, It will throw exception if the collection contains more than one item or the collection is empty. A possible usage is when querying against primary key. Only one record should be returned. Exceptional case should be when there are more than one record.
SingleOrDefault: Works on the same principal as Single except it would return null if the collection is empty.
All these methods can be added at the end of your query syntax or could receive a predicate. Like:
var db= (from c in context.Staff
where c.id == sId
select c).FirstOrDefault();
Or
var db = context.Staff.FirstOrDefault(c=> c.id == sId);
If your field id is a primary key then use SingleOrDefault.
(Old Answer
If you are trying to return particular columns from a table, then you can't project your result to your class Staff since that is generated through the framework.
You have two options. First create another class with properties same as your result set and return its object.
Or instead of selecting specific columns, select all columns and then return Staff type object.
var db= from c in context.Staff
where c.id == sId
select c;
return db;
If you want to create an other class lets say PartialStaff then you can do:
var db= from c in context.Staff
where c.id == sId
select new PartialStaff
{
Id = s.sId,
//.......rest of the properties
};
But then your method return type should be PartialStaff type collection.
When a linq query fetches a whole Entity, how can i return the result
as an object type? I mean not just a field but also to return all the
fields of that specific column, as one entity.
For example a Staff table in sql. To return a staff object entity.
You could try this one:
var db= (from c in context.Staff
where c.id == sId
select c);
or
db = context.Staff.Where(s=>s.id == sId);
The result of the above query is a sequence of Staff objects, IEnumerable<Staff>. Specifically, this sequence will contain all the Staff objects in context.Staff, whose id is equal to sId.
Gives an error because i want to return result as Staff object.
Now If you are sure that there would be at most one item in context.Staff with the given id, you could try this:
var db = (from c in context.Staff
where c.id == sId
select c).SingleOrDefault();
or
var db = context.Staff.SingleOrDefault(s=>s.id == sId);
This query will return the single Staff object with the given id and if there isn't any Staff object in context.Staff with this id, will return null.
I'm using Entity Framework in order to connect to MySQL.
I have an incremental data which implements subversion technique. Each set of subversion record has the same LinkedId and separated by UpdatedTime.
My expectation is getting the latest version of each record from database. Thus, I write a linq statement like below:
public List<Entry> LoadFinalEntries(int rptId) {
return (from ent in ctx.Entries
where ent.ReportId == rptId
orderby ent.LinkedId, ent.UpdatedTime descending
group ent by ent.LinkedId into svnEnt
select svnEnt.FirstOrDefault()).ToList();
}
But at the run-time, it throws an EntityCommandCompilationException telling that "Specified method is not supported.". I know that method is FirstOrDefault, but cannot find anyway to fix it.
Please help me to find out another way to do.
What do you think using inner query?
return (from ent in ctx.Entries
where ent.ReportId == rptId &&
ent.UpdatedTime ==
(from inner in ctx.Entries
where inner.LinkedId == ent.LinkedId &&
inner.ReportId == rptId
select inner.UpdatedTime).Max()
select ent).ToList();
I've found a better solution after referring this article
public List<Entry> GetFinalEntries(int rptId) {
// Get the latest updated time for each linked id
var latestUpdatedTimes = from ent in ctx.Entries
where ent.ReportId == rptId
group ent by ent.LinkedId into svnEnt
select new { LinkedId = svnEnt.Key, UpdatedTime = svnEnt.Max(ent => ent.UpdatedTime) };
// Compare (by joining) to get full entries
var latestEntries = from ent in ctx.Entries
where ent.ReportId == rptId
join time in latestUpdatedTimes
on new { ent.LinkedId, ent.UpdatedTime } equals time
select ent;
return latestEntries.ToList();
}
Now it works fine.
in ADO.NET I can use delete statement
to delete some rows from an SQL table.
what is the equivalent in Entity Framework?
How can I achieve that same result?
updateing with null objects isn't the same.
Replies telling you, that you need to first fetch objects (strictly speaking keys are enough, but then you need to do some work manually) into memory and mark them for deletion and finally call SaveChanges. Though that's the "normal" approach, there's bunch of extensions, helpers, ... that allow you to do i.e. batch deletes, batch updates and other helpful stuff.
You can check EntityFramework.Extended (also on GitHub) or Entity Framework Extensions (sources there as well).
You need to retrieve the object to be deleted first.
For example :
// Assuming ID is primary key in `Customer` entity
Customer cust = (from c in context.Customers where c.ID = "1" select c);
Then delete the object using DataContext.entity.DeleteObject
context.Customers.DeleteObject(cust);
context.SaveChanges();
More : DataContext
First of all you need to create instance from your Database entities,after that you should select your desired object,then delete it :
TestEntities db = new TestEntities();
Test ts = (from rows in db.Tests
where rows.ID == 1
select rows).FirstOrDefault();
if (ts != null)
{
db.Tests.DeleteObject(ts);
db.SaveChanges();
}
Update :
If your result set is a list, I mean more than one record you can use this solution :
List<Test> lst = (from rows in db.Tests select rows).ToList();
foreach (Test item in lst)
{
db.Tests.DeleteObject(item);
}
db.SaveChanges();
How to write LINQ statement that returns ROWNUMBER of element with id == something?
There is no direct way to do this that I'm aware of. You'd have to pull the whole query down to the client, and the from there you could project in the row numbers. As an alternative, you could write a stored procedure that uses ROW_NUMBER, and then hit that proc from Linq to SQL.
In your case, the only way you're going to be able to do this would be client side. Keep in mind that the following statement is NOT going to do this at the server, but will pull down your whole table and get the index at the client...
using (var dc = new DataClasses1DataContext())
{
var result = dc.Users
.AsEnumerable() // select all users from the database and bring them back to the client
.Select((user, index) => new // project in the index
{
user.Username,
index
})
.Where(user => user.Username == "sivey"); // filter for your specific record
foreach (var item in result)
{
Console.WriteLine(string.Format("{0}:{1}", item.index, item.Username));
}
}
You should be able to use the Skip and Take extension methods to accomplish this.
For example, if you want row 10:
from c in customers
where c.Region == "somewhere"
orderby c.CustomerName
select new {c.CustomerID, c.CustomerName}
.Skip(9).Take(1);
How To Project a Line Number Into Linq Query Results
How To Project a Line Number Into Linq Query Results