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Entity Framework 4: Selecting Single Record
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Closed 7 years ago.
I'm trying to understand whether it possible to get a single row from a database using entity framework without returning all the data. Maybe I'm misunderstanding how EF work but I believe its similar to the following:
TBL1
Id | Name | Place
1 | Teressa Green | UK
2 | Robin Banks | Germany
3 | Liam Neeson | Canada
If I want Robin Banks Id do something similar to
context.tbl1.where(obj => obj.name = "Robin Banks")
However from what I've understood this is getting all data from the table and then filtering down to the one row. Is there a way to return just the one row back to the logic without initially returning all the data?
To put the my issue in one sentence. I'm trying to avoid loading back all rows when I just want 1.
I think you need to use here SingleOrDefault
var result= db.yourtable
.SingleOrDefault(c=>c.Name== "Some Name");
Whenever you use SingleOrDefault, you clearly state that the query should result in at most a single result
This line will not actually execute anything on the database:
context.tbl1.Where(obj => obj.name == "Robin Banks")
It will return an IEnumerable<tbl1> which is going to be lazily evaluated when you come to use it. To execute an actual query on the database you need to perform an enumeration on the IEnumerable<tbl1> (e.g. a foreach, .ToList() or .SingleOrDefault()). At this point EF will convert your Where() clause into actual SQL and execute it on the database, returning the specified data. So, it will get all data that matches your predicate obj.name="Robin Banks". It will not get all the data in tbl1 using a SQL statement and then filter the results in .NET - that's not how it works.
However, you can do this (if you need to, but not recommended almost 100% of the time) by first enumerating with .ToList():
context.tbl1.Where(obj => <some SQL evaluated expression>).ToList()
And then adding an additional predicate on the end:
context.tbl1.Where(obj => <some SQL evaluated expression>).ToList().Where(obj => <some .NET evaluated expression>).ToList()
You can log the actual SQL being generated by EF by doing the following with your context:
context.Database.Log = Console.WriteLine;
And see for yourself what's going on under the hood.
you are not sure if an item with a given key exists --> FirstOrDefault
Entity Framework 4 Single() vs First() vs FirstOrDefault()
Related
I have few related objects and relation is like
public class Project
{
public List<ProjectEdition> editions;
}
public class ProjectEdition
{
public List<EditionItem> items;
}
public class EditionItem
{
}
I wanted to fetch the EditionItems from Last entries of ProjectEditions only for each Project
Example
Project#1 -> Edition#1 [contains few edition items ] , Edition#2 [contains few edition items]
Project#2 -> Edition#1 ,Edition#2 and Edition#3
My required output contains EditionItems from Edition#2 of Project#1 and Edition#3 of Project#2 only . I mean EditionItems from latest edition of a Project or last edition of a Project only
To get this i tried this query
List<EditionItem> master_list = context.Projects.Select(x => x.ProjectEditions.LastOrDefault())
.SelectMany(x => x.EditionItems).ToList();
But its returns error at LatsOrDefault() section
An exception of type 'System.NotSupportedException' occurred in EntityFramework.SqlServer.dll but was not handled in user code
Additional information: LINQ to Entities does not recognize the method '---------.Models.ProjectEdition LastOrDefault[ProjectEdition](System.Collections.Generic.IEnumerable`1
so how can i filter for last edition of a project and then get the list of EditionItems from it in a single LINQ call
Granit got the answer right, so I won't repeat his code. I would like to add the reasons for this behaviour.
Entity Framework is magic (sometimes too much magic) but it yet translates your LINQ queries into SQL and there are limitations to that of what your underlying database can do (SQL Server in this case).
When you call context.Projects.FirstOrDefault() it is translated into something like Select TOP 1 * from Projects. Note the TOP 1 part - this is SQL Server operator that limits number of rows returned. This is part of query optimisation in SQL Server. SQL Server does not have any operators that will give you LAST 1 - because it needs to run the query, return all the results, take the last one and dump the rest - this is not very efficient, think of a table with a couple (bi)million records.
So you need to apply whatever required sort order to your query and limit number of rows you return. If you need last record from the query - apply reverse sort order. You do need to sort because SQL Server does not guarantee order of records returned if no Order By is applied to the query - this is due to the way the data is stored internally.
When you write LINQ queries with EF I do recommend keep an eye on what SQL is generated by your queries - sometimes you'll see how complex they come out and you can easily simplify the query. And sometimes with lazy-loading enabled you introduce N+1 problem with a stroke of a key (literally). I use ExpressProfiler to watch generated SQL, LinqPad can also show you the SQL queries and there are other tools.
You cannot use method LastOrDefault() or Last() as discussed here.
Insetad, you can use OrderByDescending() in conjunction with FirstOrDefault() but first you need to have a property in you ProjectEdition with which you want to order the entities. E.g. if ProjectEdition has a property Id (which there is a good chance it does), you can use the following LINQ query:
List<EditionItem> master_list = context.Projects.Select(
x => x.ProjectEditions
.OrderByDescending(pe => pe.Id)
.FirstOrDefault())
.SelectMany(x => x.EditionItems).ToList();
List<EditionItem> master_list = context.Projects
.Select(p => p.editions.LastOrDefault())
.SelectMany(pe => pe.items).ToList();
IF LastOrDefault not supported you can try using OrderByDescending
List<EditionItem> master_list = context.Projects
.Select(p => p.editions.OrderByDescending(e => e.somefield).FirstOrDefault())
.SelectMany(pe => pe.items).ToList();
from p in context.project
from e in p.projectEdition.LastOrDefault()
select new EditionItem
{
item1 = e.item1
}
Please try this
I am using ASP NET MVC 4.5 and EF6, code first migrations.
I have this code, which takes about 6 seconds.
var filtered = _repository.Requests.Where(r => some conditions); // this is fast, conditions match only 8 items
var list = filtered.ToList(); // this takes 6 seconds, has 8 items inside
I thought that this is because of relations, it must build them inside memory, but that is not the case, because even when I return 0 fields, it is still as slow.
var filtered = _repository.Requests.Where(r => some conditions).Select(e => new {}); // this is fast, conditions match only 8 items
var list = filtered.ToList(); // this takes still around 5-6 seconds, has 8 items inside
Now the Requests table is quite complex, lots of relations and has ~16k items. On the other hand, the filtered list should only contain proxies to 8 items.
Why is ToList() method so slow? I actually think the problem is not in ToList() method, but probably EF issue, or bad design problem.
Anyone has had experience with anything like this?
EDIT:
These are the conditions:
_repository.Requests.Where(r => ids.Any(a => a == r.Student.Id) && r.StartDate <= cycle.EndDate && r.EndDate >= cycle.StartDate)
So basically, I can checking if Student id is in my id list and checking if dates match.
Your filtered variable contains a query which is a question, and it doesn't contain the answer. If you request the answer by calling .ToList(), that is when the query is executed. And that is the reason why it is slow, because only when you call .ToList() is the query executed by your database.
It is called Deferred execution. A google might give you some more information about it.
If you show some of your conditions, we might be able to say why it is slow.
In addition to Maarten's answer I think the problem is about two different situation
some condition is complex and results in complex and heavy joins or query in your database
some condition is filtering on a column which does not have an index and this cause the full table scan and make your query slow.
I suggest start monitoring the query generated by Entity Framework, it's very simple, you just need to set Log function of your context and see the results,
using (var context = new MyContext())
{
context.Database.Log = Console.Write;
// Your code here...
}
if you see something strange in generated query try to make it better by breaking it in parts, some times Entity Framework generated queries are not so good.
if the query is okay then the problem lies in your database (assuming no network problem).
run your query with an SQL profiler and check what's wrong.
UPDATE
I suggest you to:
add index for StartDate and EndDate Column in your table (one for each, not one for both)
ToList executes the query against DB, while first line is not.
Can you show some conditions code here?
To increase the performance you need to optimize query/create indexes on the DB tables.
Your first line of code only returns an IQueryable. This is a representation of a query that you want to run not the result of the query. The query itself is only runs on the databse when you call .ToList() on your IQueryable, because its the first point that you have actually asked for data.
Your adjustment to add the .Select only adds to the existing IQueryable query definition. It doesnt change what conditions have to execute. You have essentially changed the following, where you get back 8 records:
select * from Requests where [some conditions];
to something like:
select '' from Requests where [some conditions];
You will still have to perform the full query with the conditions giving you 8 records, but for each one, you only asked for an empty string, so you get back 8 empty strings.
The long and the short of this is that any performance problem you are having is coming from your "some conditions". Without seeing them, its is difficult to know. But I have seen people in the past add .Where clauses inside a loop, before calling .ToList() and inadvertently creating a massively complicated query.
Jaanus. The most likely reason of this issue is complecity of generated SQL query by entity framework. I guess that your filter condition contains some check of other tables.
Try to check generated query by "SQL Server Profiler". And then copy this query to "Management Studio" and check "Estimated execution plan". As a rule "Management Studio" generatd index recomendation for your query try to follow these recomendations.
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Why does forcing materialization using ToList() make my query orders of magnitude faster when, if anything, it should do the exact opposite?
1) Calling First() immediately
// "Context" is an Entity Framework DB-first model
var query = from x in Context.Users
where x.Username.ToLower().Equals(User.Identity.Name.ToLower())
select x;
var User = query.First();
// ** The above takes 30+ seconds to run **
2) Calling First() after calling ToList():
var query = from x in Context.Users
where x.Username.ToLower().Equals(User.Identity.Name.ToLower())
select x;
var User = query.ToList().First(); // Added ToList() before First()
// ** Now it takes < 1 second to run! **
Update and Resolution
After getting the generated SQL, the only difference is, as expected, the addition of TOP (1) in the first query. As Andyz Smith says in his answer below, the root cause is that the SQL Server optimizer, in this particular case, chooses a worse execution plan when TOP (1) is added. Thus the problem has nothing to do with LINQ (which did the right thing by adding TOP (1)) and everything to do with the idiosyncrasies of SQL Server.
I can only think of one reason...
To test it, can you please remove the Where clause and re-run the test? Comment here if the result is the first statement being faster, and i will explain why.
Edit
In the LINQ statement Where clause, you are using the .ToLower() method of the string. My guess is that LINQ does not have built in conversion to SQL for this method, so the resultant SQL is something line
SELECT *
FROM Users
Now, we know that LINQ lazy loads, but it also knows that since it has not evaluated the WHERE clause, it needs to load the elements to do the comparison.
Hypothesis
The first query is lazy loading EVERY element in the result set. It is then doing the .ToLower() comparison and returning the first result. This results in n requests to the server and a huge performance overhead. Cannot be sure without seeing the SQL Tracelog.
The Second statement calls ToList, which requests a batch SQL before doing the ToLower comparison, resulting in only one request to the server
Alternative Hypothesis
If the profiler shows only one server execution, try executing the same query with the Top 1 clause and see if it takes as long. As per this post (Why is doing a top(1) on an indexed column in SQL Server slow?) the TOP clause can sometimes mess with the SQL server optimiser and stop it using the correct indices.
Curiosity edit
try changing the LINQ to
var query = from x in Context.Users
where x.Username.Equals(User.Identity.Name, StringComparison.OrdinalIgnoreCase)
select x;
Credit to #Scott for finding the way to do case insensitive comparison in LINQ. Give it a go and see if it is faster.
The SQL won't be the same as Linq is lazy loading. So your call to .ToList() will force .Net to evaluate the expression, then in memory select the first() item.
Where as the other option should add top 1 into the SQL
E.G.
var query = from x in Context.Users
where x.Username.ToLower().Equals(User.Identity.Name.ToLower())
select x;
//SQL executed here
var User = query.First();
and
var query = from x in Context.Users
where x.Username.ToLower().Equals(User.Identity.Name.ToLower())
select x;
//SQL executed here!
var list = query.ToList();
var User = query.First();
As below, the first query should be faster! I would suggest doing a SQL profiler to see what's going on. The speed of the queries will depend on your data structure, number of records, indexes, etc.
The timing of your test will alter the results also. As a couple of people have mentioned in comments, the first time you hit EF it needs to initialise and load the metadata. so if you run these together, the first one should always be slow.
Here's some more info on EF performance considerations
notice the line:
Model and mapping metadata used by the Entity Framework is loaded into
a MetadataWorkspace. This metadata is cached globally and is available
to other instances of ObjectContext in the same application domain.
&
Because an open connection to the database consumes a valuable
resource, the Entity Framework opens and closes the database
connection only as needed. You can also explicitly open the
connection. For more information, see Managing Connections and
Transactions in the Entity Framework.
So, the optimizer chooses a bad way to run the query.
Since you can't add optimizer hints to the SQL to force the optimizer to choose a better plan I see two options.
Add a covering index/indexed view on all the columns that are retrieved/included in the select Pretty ludicrous, but I think it will work, because that index will make it easy peasy for the optimizer to choose a better plan.
Always prematurely materialize queries that include First or Last or Take. Dangerous because as the data gets larger the break even point between pulling all the data locally and doing the First() and doing the query with Top on the server is going to change.
http://geekswithblogs.net/Martinez/archive/2013/01/30/why-sql-top-may-slow-down-your-query-and-how.aspx
https://groups.google.com/forum/m/#!topic/microsoft.public.sqlserver.server/L2USxkyV1uw
http://connect.microsoft.com/SQLServer/feedback/details/781990/top-1-is-not-considered-as-a-factor-for-query-optimization
TOP slows down query
Why does TOP or SET ROWCOUNT make my query so slow?
Using the Entity Framework, when one executes a query on lets say 2000 records requiring a groupby and some other calculations, does the query get executed on the server and only the results sent over to the client or is it all sent over to the client and then executed?
This using SQL Server.
I'm looking into this, as I'm going to be starting a project where there will be loads of queries required on a huge database and want to know if this will produce a significant load on the network, if using the Entity Framework.
I would think all database querying is done on the server side (where the database is!) and the results are passed over. However, in Linq you have what's known as Delayed Execution (lazily loaded) so your information isn't actually retrieved until you try to access it e.g. calling ToList() or accessing a property (related table).
You have the option to use the LoadWith to do eager loading if you require it.
So in terms of performance if you only really want to make 1 trip to the Database for your query (which has related tables) I would advise using the LoadWith options. However, it does really depend on the particular situation.
It's always executed on SQL Server. This also means sometimes you have to change this:
from q in ctx.Bar
where q.Id == new Guid(someString)
select q
to
Guid g = new Guid(someString);
from q in ctx.Bar
where q.Id == g
select q
This is because the constructor call cannot be translated to SQL.
Sql's groupby and linq's groupby return differently shaped results.
Sql's groupby returns keys and aggregates (no group members)
Linq's groupby returns keys and group members.
If you use those group members, they must be (re-)fetched by the grouping key. This can result in +1 database roundtrip per group.
well, i had the same question some time ago.
basically: your linq-statement is converted to a sql-statement. however: some groups will get translated, others not - depending on how you write your statement.
so yes - both is possible
example:
var a = (from entity in myTable where entity.Property == 1 select entity).ToList();
versus
var a = (from entity in myTable.ToList() where entity.Property == 1 select entity).ToList();
I have 2 related Linq to SQL questions. Please see the image below to see what my Model looks like.
Question 1
I am trying to figure how to eager load the User.AddedByUser field on my User class/table. This field is generated from the relationship on the User.AddedByUserId field. The table is self-referencing, and I am trying to figure out how to get Linq to SQL to load up the User.AddedByUser property eagerly, i.e. whenever any User entity is loaded/fetched, it must also fetch the User.AddedByUser and User.ChangedByUser. However, I understand that this could become a recursive problem...
Update 1.1:
I've tried to use the DataLoadOptions as follows:
var options = new DataLoadOptions();
options.LoadWith<User>(u => u.ChangedByUser);
options.LoadWith<User>(u => u.AddedByUser);
db = new ModelDataContext(connectionString);
db.LoadOptions = options;
But this doesn't work, I get the following exception on Line 2:
System.InvalidOperationException occurred
Message="Cycles not allowed in LoadOptions LoadWith type graph."
Source="System.Data.Linq"
StackTrace:
at System.Data.Linq.DataLoadOptions.ValidateTypeGraphAcyclic()
at System.Data.Linq.DataLoadOptions.Preload(MemberInfo association)
at System.Data.Linq.DataLoadOptions.LoadWith[T](Expression`1 expression)
at i3t.KpCosting.Service.Library.Repositories.UserRepository..ctor(String connectionString) in C:\Development\KP Costing\Trunk\Code\i3t.KpCosting.Service.Library\Repositories\UserRepository.cs:line 15
InnerException:
The exception is quite self-explanatory - the object graph isn't allowed to be Cyclic.
Also, assuming Line 2 didn't throw an exception, I'm pretty sure Line 3 would, since they are duplicate keys.
Update 1.2:
The following doesn't work either (not used in conjuction with Update 1.1 above):
var query = from u in db.Users
select new User()
{
Id = u.Id,
// other fields removed for brevityy
AddedByUser = u.AddedByUser,
ChangedByUser = u.ChangedByUser,
};
return query.ToList();
It throws the following, self-explanatory exception:
System.NotSupportedException occurred
Message="Explicit construction of entity type 'i3t.KpCosting.Shared.Model.User' in query is not allowed."
I am now REALLY at a loss on how to solve this. Please help!
Question 2
On every other table in my DB, and hence Linq to SQL model, I have two fields, Entity.ChangedByUser (linked to Entity.ChangedByUserId foreign key/relationship) and Entity.AddedByUser (linked to Entity.AddedByUserId foreign key/relationship)
How do I get Linq to SQL to eageryly load these fields for me? Do I need to do a simple join on my queries?, or is there some other way?
Linq to SQL eager loading on self referencing table http://img245.imageshack.us/img245/5631/linqtosql.jpg
Any type of cycles just aren't allowed. Since the LoadWith<T> or AssociateWith<T> are applied to every type on the context, there's no internal way to prevent an endless loop. More accurately, it's just confused on how to create the SQL since SQL Server doesn't have CONNECT BY and CTEs are really past what Linq can generate automatically with the provided framework.
The best option available to you is to manually do the 1 level join down to the user table for both of the children and an anonymous type to return them. Sorry it's not a clean/easy solution, but it's really all that's available thus far with Linq.
Maybe you could try taking a step back and seeing what you want to do with the relation? I'm assuming you want to display this information to the user in e.g. "modified by Iain Galloway 8 hours ago".
Could something like the following work? :-
var users = from u in db.Users
select new
{
/* other stuff... */
AddedTimestamp = u.AddedTimestamp,
AddedDescription = u.AddedByUser.FullName,
ChangedTimestamp = u.ChangedTimestamp,
ChangedDescription = u.ChangedByUser.FullName
};
I've used an anonymous type there for (imo) clarity. You could add those properties to your User type if you preferred.
As for your second question, your normal LoadWith(x => x.AddedByUser) etc. should work just fine - although I tend to prefer storing the description string directly in the database - you've got a trade-off between your description updating when ChangedByUser.FullName changes and having to do something complicated and possibly counterintuitive if the ChangedByUser gets deleted (e.g. ON DELETE CASCADE, or dealing with a null ChangedByUser in your code).
Not sure there is a solution to this problem with Linq to Sql. If you are using Sql Server 2005 you could define a (recursive like) Stored Procecdure that uses common table expressions to get the result that you want and then execute that using DataContext.ExecuteQuery.